• Written informed consent obtained to participate in the study and Health Insurance Portability and Accountability Act (HIPAA) authorization for release of personal health information to the sponsor, sites, and relevant study organizations.

• Age ≥ 18 years at the time of consent.

• Karnofsky scale ≥ 50% or Eastern Cooperative Oncology Group (ECOG) ≤ 2.

• Histological evidence of B-cell PTLD (any histologic subtype) following solid organ transplantation; expresses CD20; with or without EBV association.

• Treatment failure of immunosuppression reduction (ISR). NOTE: if ISR was deemed not feasible by treating physician, ISR treatment failure may be waived.

• Treatment failure of rituximab or rituximab plus any concurrent or sequentially administered chemotherapy regimen.

• Measurable disease of \> 1.5 cm in diameter and/or bone marrow involvement.

• Subjects having undergone heart, lung, liver, kidney, pancreas, small intestine transplantation or a combination of the organ transplantations mentioned.

• HIV infection is allowed if viral load is undetectable at time of enrollment, CD4+ count \> 200 cells/uL, and subject remains on anti-viral therapy.

• Resolution of toxicities from prior therapy to a grade that does not contraindicate trial participation in the opinion of the investigator.

• Expected survival greater than 60 days.

• Absolute neutrophil count 1.0 ≥ x 10\^9/L.

• Platelets 50 ≥ x 10\^9/L.

• Creatinine clearance (mL/min) ≥ 30 mL/min - Cockcroft-Gault Equation.

⁃ Note: Hematology and other lab parameters that are ≤ grade 2 BUT still meet criteria for study entry are allowed. Furthermore, changes in laboratory parameters during the study should not be considered adverse events unless they meet criteria for dose modification(s) of study medication outlined by the protocol and/or worsen from baseline during therapy.

• Bilirubin ≤ 3.0 x upper limit of normal (ULN). Subjects with Gilbert's syndrome may be enrolled despite a total bilirubin level \> 3.0 mg/dL if their conjugated bilirubin is ≤ 3.0 × ULN).

⁃ Note: Hematology and other lab parameters that are ≤ grade 2 BUT still meet criteria for study entry are allowed. Furthermore, changes in laboratory parameters during the study should not be considered adverse events unless they meet criteria for dose modification(s) of study medication outlined by the protocol and/or worsen from baseline during therapy.

• Aspartate aminotransferase (AST) ≤ 3.0 x ULN.

⁃ Note: Hematology and other lab parameters that are ≤ grade 2 BUT still meet criteria for study entry are allowed. Furthermore, changes in laboratory parameters during the study should not be considered adverse events unless they meet criteria for dose modification(s) of study medication outlined by the protocol and/or worsen from baseline during therapy.

• Alanine aminotransferase (ALT) ≤ 3.0 x ULN.

⁃ Note: Hematology and other lab parameters that are ≤ grade 2 BUT still meet criteria for study entry are allowed. Furthermore, changes in laboratory parameters during the study should not be considered adverse events unless they meet criteria for dose modification(s) of study medication outlined by the protocol and/or worsen from baseline during therapy.

• Females of childbearing potential must have a negative serum pregnancy test within 3 days prior to registration. NOTE: Females are considered of childbearing potential unless they are surgically sterile (have undergone a hysterectomy, bilateral tubal ligation, or bilateral oophorectomy) or they are naturally postmenopausal for at least 12 consecutive months. Documentation of postmenopausal status must be provided.

• Females of childbearing potential must be willing to abstain from heterosexual activity or to use 2 forms of effective methods of contraception from the time of informed consent until 12 months after treatment the last dose of epcoritamab. The two contraception methods can be comprised of two barrier methods, or a barrier method plus a hormonal method or an intrauterine device.

• Male subjects with female partners must have had a prior vasectomy or agree to use an adequate method of contraception (i.e., double barrier method: condom plus spermicidal agent) starting with the first dose of study therapy through 12 months after the last dose of epcoritamab.

• Subjects with prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the experimental regimen are eligible for the trial.

• Subject is willing and able to comply with study procedures based on the judgement of the investigator or protocol designee.