First-in-Human (FIH) Trial in Patients With Relapsed, Progressive or Refractory B-Cell Lymphoma

Main Inclusion Criteria - Escalation Part (recruitment completed) * Documented CD20+ mature B-cell neoplasm 1. DLBCL - de novo or transformed 2. HGBCL 3. PMBCL 4. FL 5. MCL 6. SLL 7. MZL (nodal, extranodal or mucosa associated) * Relapsed and/or refractory disease following treatment with an anti-CD20 monoclonal antibody (e.g. rituximab) potentially in combination with chemotherapy and/or relapsed after autologous stem cell rescue. * Eastern Cooperative Oncology Group (ECOG) performance status 0,1 or 2. * Participants must have measurable disease by computed tomography (CT), magnetic resonance imaging (MRI) or Positron emission tomography-Computed tomography (PET-CT) scan * Acceptable renal function. * Acceptable liver function. Main Inclusion Criteria - Expansion \& Dose-OPT Parts * Documented CD20 positive mature B cell neoplasm or CD20+ MCL. * DLBCL, de novo or transformed (including double hit or triple hit). * PMBCL * FL grade 3B * Histologic confirmed FL * MZL * SLL * MCL (prior Bruton's tyrosine kinase inhibitor \[BTKi\] or intolerant to BTKi) * At least 2 therapies including an anti-CD20 monoclonal antibody containing chemotherapy combination regimen. * Either failed prior autologous hematopoietic stem cell transplantation (HSCT) or ineligible for autologous stem cell transplantation due to age or comorbidities. * At least 1 measurable site of disease based on CT, MRI or PET-CT scan with involvement of 2 or more clearly demarcated lesions and or nodes. Main Exclusion Criteria - All Parts * Primary central nervous system (CNS) lymphoma or CNS involvement by lymphoma at screening. * Known past or current malignancy other than inclusion diagnosis. * Aspartate aminotransferase (AST), and/or alanine transaminase (ALT) \>3 × upper limit of normal. * Total bilirubin \>1.5 × upper limit of normal, unless bilirubin rise is due to Gilbert's syndrome or of non-hepatic origin. * Estimated Creatinine clearance (CrCl) \<45 milliliters (mL)/min. * Known clinically significant cardiovascular disease. * Ongoing active bacterial, viral, fungal, mycobacterial, parasitic, or other infection requiring systemic treatment (excluding prophylactic treatment). Past coronavirus disease 2019 (COVID-19) infection may be a risk factor. * Confirmed history or current autoimmune disease or other diseases resulting in permanent immunosuppression or requiring permanent immunosuppressive therapy. * Seizure disorder requiring therapy (such as steroids or anti-epileptics). * Any prior therapy with an investigational bispecific antibody targeting CD3 and CD20. * Prior treatment with chimeric antigen receptor T-cell (CAR-T) therapy within 30 days prior to first epcoritamab administration. * Eligible for curative intensive salvage therapy followed by high dose chemotherapy with HSCT rescue. * Autologous HSCT within 100 days prior to first epcoritamab administration, or any prior allogeneic HSCT or solid organ transplantation. * Active hepatitis B (deoxyribonucleic acid \[DNA\] polymerase chain reaction \[PCR\]-positive) or hepatitis C (ribonucleic acid \[RNA\] PCR-positive infection). Participants with evidence of prior hepatitis B (HBV) but who are PCR-negative are permitted in * Known human immunodeficiency virus (HIV) infection. * Exposed to live or live attenuated vaccine within 4 weeks prior to signing Informed consent form (ICF). * Pregnancy or breast feeding. * Participant is known or suspected of not being able to comply with the study protocol or has any condition for which, participation would not be in the best interest of the participant. * Contraindication to all uric acid lowering agents. NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.