A Study to Assess the Efficacy, Safety, Pharmacodynamics, and Pharmacokinetics of Tazemetostat in Combination With Lenalidomide Plus Rituximab Versus Placebo in Combination With Lenalidomide Plus Rituximab in Adult Patients at Least 18 Years of Age With Relapsed/Refractory Follicular Lymphoma.

Inclusion Criteria: 1. Have voluntarily agreed to provide written informed consent and demonstrated willingness and ability to comply with all aspects of the protocol. 2. Males or females are ≥18 years of age, or per country adult legal age regulations, at the time of providing voluntary written informed consent. 3. Life expectancy ≥3 months before enrollment. 4. Meet requirement for hepatitis and human immunodeficiency virus (HIV) infection as follows * Negative serologic or polymerase chain reaction (PCR) test results for acute or chronic hepatitis B virus (HBV) infection Note: Participants whose HBV infection status could not be determined by serologic test results have to be negative for HBV-DNA by PCR to be eligible for study participation. Participants seropositive for HBV with undetectable HBV DNA by PCR are permitted with appropriate antiviral prophylaxis. * Negative test results for hepatitis C virus (HCV) Note: Participants who are positive for HCV antibody must be negative for HCV RNA by PCR to be eligible for study participation * If HIV positive, HIV infection is controlled. Based on Cancer Clinical Trial Eligibility Criteria: Patients with HIV, Hepatitis B Virus, or Hepatitis C Virus Infections - Guidance for Industry (https://www.fda.gov/media/121319/download), patients with HIV should be considered eligible if they have CD4+ T-cell counts ≥ 350 cells/uL and in general, if they have not had an opportunistic infection within the past 12 months. Other exclusion criteria should be considered regarding the drug-drug interaction if antiviral drugs are used. Therefore, in case of controlled HIV infection, since antiviral drugs are used, trial patients should be on established ART for at least 4 weeks and have an HIV viral load less than 400 copies/mL prior to enrolment. 5. Have histologically confirmed FL, Grades 1 to 3A. 6. Must have been previously treated with at least 1 prior systemic chemotherapy, immunotherapy, or chemoimmunotherapy: a. Systemic therapy includes treatments such as: i. Rituximab monotherapy ii. Chemotherapy given with or without rituximab iii. Radioimmunoconjugates such as 90Y-ibritumomab tiuxetan and 131I-tositumomab. b. Systemic therapy does not include, for example: i. Local involved field radiotherapy for limited-stage disease ii. Helicobacter pylori eradication c. Prior investigational therapies will be allowed provided the subject has received at least 1 prior systemic therapy as discussed in Inclusion Criterion #6a. d. Prior autologous/allogeneic hematopoietic stem cell transplant (HSCT) will be allowed. e. Prior chimeric antigen receptor T-cell therapy (CAR T) will be allowed. 7. Must have documented relapsed, refractory, or PD after treatment with systemic therapy (refractory defined as less than PR or disease progression \<6 months after last dose). 8. Have measurable disease as defined by the Lugano Classification (Cheson, 2014; Appendix 5). 9. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2. 10. Within 7 days prior to randomization, all clinically significant toxicity related to a prior anticancer treatment (ie, chemotherapy, immunotherapy, and/or radiotherapy must have either resolved to Grade 1 per NCI CTCAE Version 5.0 OR are clinically stable and no longer clinically significant. 11. Have provided sufficient tumor tissue block or unstained slides for EZH2 mutation testing in all subjects to allow for stratification a. If EZH2 mutation status is known from site-specific testing, subjects can be enrolled. Tumor tissue will be required for confirmatory testing of EZH2 status at study-specific laboratories. If the archival tumor sample was collected more than 24 months prior to the anticipated administration of the first dose (cycle 1 day 1), then a fresh biopsy must be provided. Fresh tumor biopsy is appropriate except for procedures deemed to result in unacceptable risk because of the anatomical location including brain, lung/mediastinum, pancreas, or endoscopic procedures extending beyond the esophagus, stomach, or bowel. Archival tumor biopsy sections mounted on slides are also acceptable. NOTE: Confirmatory testing will also be performed for Stage 1, if local EZH2 testing is conducted, unless there is insufficient tumor tissue to perform testing after discussion with the Sponsor's or Designee Medical Monitor. 12. Time between prior anticancer therapy and first dose of tazemetostat as follows: 1. Cytotoxic chemotherapy - At least 21 days. 2. Noncytotoxic chemotherapy (eg, small molecule inhibitor) - At least 14 days. 3. Nitrosoureas - At least 6 weeks. 4. Monoclonal and/or bispecific antibodies or CAR T - At least 28 days. 5. Radiotherapy - At least 6 weeks from prior radioisotope therapy; at least 12 weeks from 50% pelvic or total body irradiation. 13. Adequate renal function defined as calculated creatinine clearance ≥30 mL/minute per the Cockcroft and Gault formula. 14. Adequate bone marrow function: a. Absolute neutrophil count (ANC) ≥1000/mm3 (≥1.0 × 10\^9/L) if no lymphoma infiltration of bone marrow OR ANC ≥750/mm3 (≥75 × 10\^9/L) with bone marrow infiltration * Without growth factor support (filgrastim or pegfilgrastim) for at least 14 days. b. Platelets ≥75,000/mm3 (≥75 × 10\^9/L) * Evaluated at least 7 days after last platelet transfusion. c. Hemoglobin ≥9.0 g/dL * May receive transfusion 15. Adequate liver function: 1. Total bilirubin ≤1.5 × the upper limit of normal (ULN) except for unconjugated hyperbilirubinemia of Gilbert's syndrome. 2. Alkaline phosphatase (ALP) (in the absence of bone disease), alanine aminotransferase (ALT), and aspartate aminotransferase (AST) ≤3 × ULN (≤5 × ULN if subject has liver infilration). 16. International normalized ratio (INR) ≤1.5 × ULN and activated partial thromboplastin time (aPTT) ≤1.5 × ULN (unless on warfarin, then INR ≤3.0). In subjects with thromboembolism risk, prophylactic anticoagulation, or antiplatelet therapy at investigator discretion is recommended. 17. Females of childbearing potential (FCBP) must have a negative urine or serum pregnancy tests (beta-human chorionic gonadotropin \[β-hCG\] tests with a minimum sensitivity of 25 mIU/mL or equivalent units of β-hCG) at screening within 10 to 14 days prior to first dose of study drug. The subject may not receive study drug until the study doctor has verified that the results of pregnancy tests are negative. All females will be considered to be of childbearing potential unless they are naturally postmenopausal (at least 24 months consecutively amenorrhoeic \[amenorrhea following cancer therapy does not rule out childbearing potential\] and without other known or suspected cause) or have been sterilized surgically (ie, total hysterectomy and/or bilateral oophorectomy, with surgery completed at least 1 month before dosing). 18. Females of childbearing potential (FCBP) enrolled must either practice complete abstinence or agree to use two reliable methods of contraception simultaneously. This includes ONE highly effective method of contraception and ONE additional effective contraceptive method. Contraception must begin at least 28 days prior to first dose of study drug, continue during study treatment (including during dose interruptions), and for 12 months after study drug discontinuation. Female subjects must also refrain from breastfeeding for 12 months following last dose of study drug. If the below contraception methods are not appropriate for the FCBP, she must be referred to a qualified contraception provider to determine the medically effective contraception method appropriate for the subject. The following are examples of highly effective and additional effective methods of contraception: Examples of highly effective methods: * Intrauterine device (IUD) * Hormonal (ovulation inhibitory combined \[estrogen and progesterone\] birth control pills or intravaginal/transdermal system, injections, implants, levonorgestrel-releasing intrauterine system \[IUS\], medroxyprogesterone acetate depot injections, ovulation inhibitory progesterone-only pills \[e.g. desogestrel\]) NOTE: There is a potential for tazemetostat interference with hormonal contraception methods due to enzymatic induction. * Bilateral tubal ligation * Partner's vasectomy (if medically confirmed \[azoospermia\] and sole sexual partner). Examples of additional effective methods: * Male latex or synthetic condom, * Diaphragm, * Cervical Cap NOTE: Female subjects of childbearing potential exempt from these contraception requirements are subjects who practice complete abstinence from heterosexual sexual contact. True abstinence is acceptable when this is in line with the preferred and usual lifestyle of the subject. Periodic abstinence (eg, calendar, ovulation, symptothermal, or post ovulation methods) and withdrawal are not acceptable methods of contraception. 19. All study participants enrolled must be registered into the applicable pregnancy prevention program (e.g. REVLIMID REMS in the US, Pregnancy Prevention Programme \[PPP\] in Europe) for lenalidomide to be administered and be willing and able to comply with the requirements of the applicable program as appropriate for the country in which the drug is being used. a. Female subjects of childbearing potential (FCBP) must adhere to the scheduled pregnancy testing as required in theapplicable pregnancy prevention program. During study treatment, FCBP must agree to have pregnancy testing weekly for the first 28 days of study participation and then every 28 days for FCBP with regular or no menstrual cycles OR every 14 days for FCBP with irregular menstrual cycles. FCBP must also have a pregnancy test at end of lenalidomide treatment, at day 14 (for FCBP with irregular menstrual cycles) and day 28 following the last dose of lenalidomide and at overall treatment discontinuation (at the End-of-Treatment/30-day safety Follow-up visit). Female subjects exempt from this requirement are subjects who have been naturally postmenopausal for at least 24 consecutive months OR are surgically sterilized (ie, total hysterectomy or bilateral oophorectomy) with surgery at least 1 month before the first dose of study treatment. 20. Male subjects must either practice complete abstinence or agree to use a latex or synthetic condom, even with a successful vasectomy (medically confirmed azoospermia), during sexual contact with a pregnant female or FCBP from first dose of study drug, during study treatment (including during dose interruptions), and for 3 months after study drug discontinuation. NOTE: Male subjects must not donate semen or sperm from first dose of study drug, during study treatment (including during dose interruptions), and for 3 months after study drug discontinuation. Exclusion Criteria: All Subjects 1. Prior exposure to tazemetostat or other inhibitor(s) of EZH2. 2. Prior exposure to lenalidomide or drugs of the same class. 3. Grade 3b, mixed histology, or FL that has histologically transformed to diffuse large B-cell lymphoma (DLBCL) (subjects transformed from DLBCL to FL may be enrolled). 4. Has thrombocytopenia, neutropenia, or anemia of Grade ≥3 (per CTCAE Version 5.0 criteria) or any prior history of myeloid malignancies, including myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML) or myeloproliferative neoplasm (MPN). 5. Has a prior history of T-cell lymphoblastic lymphoma (T-LBL)/T-cell acute lymphoblastic leukemia (T-ALL) or B-cell acute lymphoblastic leukemia (B-ALL). 6. Subjects with uncontrolled leptomeningeal metastases or brain metastases or history of previously treated brain metastases. 7. Subjects taking medications that are known strong CYP3A inhibitors and strong or moderate CYP3A inducers (including St. John's wort). 8. Are unwilling to exclude grapefruit juice, Seville oranges, and grapefruits from the diet and/or consumed within 1 week of the first dose of study drug and for the duration of the study. 9. Major surgery within 4 weeks before the first dose of study drug. a. Note: Minor surgery (eg, minor biopsy of extracranial site, central venous catheter placement, shunt revision) is permitted within 3 weeks prior to enrollment. 10. Are unable to take oral medication OR have malabsorption syndrome or any other uncontrolled gastrointestinal condition (eg, nausea, diarrhea, vomiting) that might impair the bioavailability of tazemetostat. 11. Significant cardiovascular impairment: history of congestive heart failure greater than New York Heart Association (NYHA) Class II, uncontrolled arterial hypertension, unstable angina, myocardial infarction, or stroke within 6 months of the first dose of study drug; or cardiac ventricular arrhythmia. 12. Prolongation of corrected QT interval using Fridericia's formula (QTcF) to ≥480 msec at screening or history of long QT syndrome. 13. Venous thrombosis or pulmonary embolism within the last 3 months before starting tazemetostat. a. Note: Participants who have experienced deep vein thrombosis/pulmonary embolism more than 3 months before enrollment are eligible but are recommended to receive prophylaxis. 14. Have an active infection requiring systemic therapy. 15. Known hypersensitivity to any component of tazemetostat or lenalidomide; known severe hypersensitivity to any component of rituximab requiring hospitalization or resuscitation. 16. Active viral infection with or seropositive for HBV: HBV surface antigen (HBsAg) positive OR HBsAg negative, anti-HBs positive and/or anti-HBc positive with detectable HBV DNA. NOTE: Subjects who are HBsAg negative, anti-HBs positive and/or anti-HBc positive, but with undetectable viral DNA and normal ALT are eligible. Subjects who are seropositive due to HBV vaccination (HBsAg negative, HBV surface antibody \[anti-HBs\] positive, and HBV core antibody \[anti-HBc\] negative) are eligible. 17. Active viral infection with hepatitis C virus (as measured by positive HCV antibody and detectable viral RNA, HIV), or known active infection with human T-cell lymphotropic virus. NOTE: Subjects with a history of hepatitis C infection (HCV antibody reactive) who have normal ALT and undetectable HCV RNA are eligible. 18. Any other medical or social condition that, in the Investigator's judgment, will interfere with a participant's ability to provide informed consent, to receive study drugs, or meet study demands, or that substantially increases the risk associated with the subject's participation in the study, or that may interfere with interpretation of results. 19. Female subjects who are pregnant or lactating/breastfeeding. 20. Subjects who have undergone a solid organ transplant. 21. Subjects with malignancies other than FL. a. Exception: Subjects with another malignancy who have been disease-free for 3 years, or subjects with a history of a completely resected non-melanoma skin cancer or successfully treated in situ carcinoma are eligible.