Inclusion Criteria:
* Clinicopathological diagnosis of WM per IWWM2 criteria.
* Meeting criteria for treatment per IWWM2 criteria.
* Relapsed or refractory WM with at least 1 prior line of treatment, including an anti-CD20 monoclonal antibody containing regimen or a BTK inhibitor.
* Patients should have received a prior BTK inhibitor (except for contraindications such as bulky disease, amyloidosis, significant medication interactions, or a history of severe bleeding).
* Participants with suspected or symptomatic hyperviscosity (e.g. nosebleeds, headaches, blurred vision) must undergo plasmapheresis prior to treatment initiation.
* Adults aged ≥18
* ECOG performance status ≤2 (Karnofsky ≥60%, see Appendix A)
* A female participant of childbearing potential must have a negative highly sensitive serum pregnancy test at screening and again either a serum or urine pregnancy test within 24 hours of the start of study treatment and must agree to further serum or urine pregnancy tests during the study.
* A female participant must be:
* Not of childbearing potential, or
* Of childbearing potential and practicing at least 1 highly effective method of contraception
* A male participant must wear a condom (with or without spermicidal foam/gel/film/ cream/suppository) when engaging in any activity that allows for passage of ejaculate to another person during the study and for 3 months after receiving the last dose of study treatment.
* Participants must meet the following organ and marrow function as defined below:
* Absolute neutrophil count ≥500/mcL; the patient may enroll below this threshold if neutropenia is believed to be caused by WM bone marrow involvement. Growth factors are not permitted \<14 days prior to C1D1.
* Platelets ≥30,000/mcL believed to be caused by WM bone marrow involvement. Platelet transfusions are not permitted \<14 days prior to C1D1.
* Hemoglobin ≥ 8 g/dL. RBC transfusions are not permitted \<14 days prior to C1D1.
* Total bilirubin ≤ 1.5 X institutional ULN, or ≤3 x institutional ULN with documented liver metastases and/or Gilbert's Disease
* AST(SGOT)/ALT(SGPT) ≤2.5 × institutional upper limit of normal
* Creatinine clearance ≥30 mL/min using the Cockcroft-Gault formula
* Ability to adhere to the study visit schedule and other protocol requirements.
* Ability to understand and the willingness to sign a written informed consent document.
Exclusion Criteria:
* Received any prior BCMA-directed therapy.
* Any serious medical condition, laboratory abnormality, uncontrolled intercurrent illness, or psychiatric illness/social condition that would prevent the participant from signing the informed consent form.
* Participants who are receiving any other investigational agents for this condition.
* Participants with known CNS lymphoma.
* Female participants who are pregnant, breastfeeding, or planning to become pregnant or breastfeed while enrolled in this study.
* History of HIV infection or active hepatitis B (chronic or acute) or hepatitis C infection.
* Patients with a history of HIV infection that is well controlled on antiretroviral therapy are eligible if all of the following criteria are met: (1) undetectable HIV viral load by standard clinical assay AND (2) CD4+ T cell count of \>/=200 cells/microliter).
* Participants with occult or prior HBV infection (defined as positive total hepatitis B core antibody \[HBcAb\] and negative HBsAg) may be included if HBV DNA is undetectable, and if the participant is willing to take appropriate anti-viral prophylaxis as indicated and HBV DNA monitoring on study.
* Patients positive for hepatitis C virus (HCV) antibody are eligible only if polymerase chain reaction (PCR) is negative for HCV RNA.
* Note: Participants with serologic evidence of prior vaccination to HBV (i.e., HBs Ag-, and anti- HBs+ and anti-HBC-) and positive anti-HBc from IVIG may participate.
* Significant cardiovascular disease defined as:
* Unstable angina within the past 6 months, or
* History of myocardial infarction within the past 6 months
* Any Class 3 or 4 cardiac disease as defined by the New York Heart Association Functional Classification, or
* Uncontrolled or symptomatic arrhythmias
* Concurrent systemic immunosuppressant therapy. Systemic steroids at doses \<20mg prednisone per day are permitted.
* Concurrent systemic anti-Waldenstrom therapy.
* Active and/or ongoing autoimmune anemia and/or autoimmune thrombocytopenia (eg, idiopathic thrombocytopenia purpura).
* Vaccinated with live, attenuated vaccines within 4 weeks of first dose of study drug.
* Active uncontrolled systemic bacterial, viral, fungal or parasitic infection (except for fungal nail infection), or other clinically significant active disease process which in the opinion of the Investigator and the Sponsor makes it undesirable for the patient to participate in the trial. Screening for chronic conditions is not required.
* Major surgery within 4 weeks of first dose of study drug.
* Participants with a known hypersensitivity to any of the excipients of Teclistamab.
* Participants with a history of non-compliance to medical regimens, which will render the administration of study drug hazardous or obscure the interpretation of toxicity or AEs.
* History of a non-lymphoma malignancy, except adequately treated local basal cell or squamous cell carcinoma of the skin, cervical carcinoma in situ, superficial bladder cancer, localized prostate cancer, other adequately treated stage 1 or 2 cancer currently in complete remission, or any other cancer that is in a complete remission.
* Ongoing alcohol or drug addiction or any psychiatric condition(s) which would compromise ability to comply with study procedures.
