⁃ Patients must meet the following criteria for study entry:

• Histologically diagnosed marginal zone lymphoma (any subtypes)

• o Low grade or indolent B-cell lymphoma unclassifiable is also eligible

• Have received at least 1 prior treatment including CD20 monoclonal antibody

• Patients have relapsed or refractory disease to their most recent regimen

• Stage II, III or IV disease

• Age ≥18 years. Because no dosing or adverse event data are currently available on the use of mosunetuzumab and zanubrutinib combination in patients \<18 years of age, children are excluded from this study.

• Performance status ≤2 on the ECOG scale (≤3 if due to lymphoma)

• Requiring systemic therapy assessed by investigator based on tumor size, symptoms and/or GELF criteria

‣ A nodal or extranodal (except spleen) mass \> 7 cm in its greater diameter

⁃ At least 3 nodal or extranodal sites ≥ 3 cm in diameter

⁃ Presence of at least one B symptom

⁃ Fever (\>38 C), night sweats, weight loss \> 10% in the past 6 months

⁃ Symptomatic splenomegaly (or size \>13cm)

⁃ Impending organ compression or involvement (ureteral, orbital, gastrointestinal)

⁃ Any of the following cytopenias due to bone marrow involvement of lymphoma

• Hemoglobin ≤ 10 g/dL

∙ Platelets ≤ 100 x 109/L

∙ Absolute neutrophil count (ANC) \< 1.5x109/L

⁃ Pleural effusion or ascites

⁃ LDH \> ULN or β2 microglobulin \> ULN

• Bi-dimensionally measurable disease, with at least one nodal lesion ≥ 1.5 cm or one extra-nodal lesion \> 1 cm in longest diameter by CT, PET/CT, and/or MRI

‣ Subjects with splenic MZL who do not meet the radiographically measurable disease criteria are eligible for participation if spleen is enlarged over 16 cm and MZL is histologically confirmed by bone marrow biopsy or peripheral blood flow cytometry

⁃ Subjects with EMZL such as skin of conjunctival EMZL who do not meet the radiographically measurable disease criteria are eligible for participation if at least one of the skin lesions is histologically confirmed as MZL and measures ≥1.5 cm in diameter

• Patients must have adequate organ and marrow function as defined below:

‣ Total bilirubin ≤ 1.5 institutional ULN, unless consistent with Gilbert's (ratio between total and direct bilirubin \> 5)

⁃ AST (SGOT) and ALT (SGPT) ≤ 3 x institutional ULN

⁃ Alkaline phosphatase \< 2.5 ULN

⁃ Creatinine clearance ≥ 40 ml/min calculated by modified Cockcroft-Gault formula

⁃ Blood counts below if without lymphoma cause for cytopenia

⁃ Hemoglobin ≥ 8 g/dL

⁃ Platelets ≥ 75 x 109/L

⁃ Absolute neutrophil count (ANC) ≥ 1.0x109/L

⁃ Blood counts below if cytopenia are due to lymphoma (such as bone marrow involvement or splenomegaly)

• Hemoglobin ≥ 6 g/dL (no transfusion within 7 days prior to treatment)

∙ Platelets ≥ 50 x 109/L

∙ Absolute neutrophil count (ANC) ≥ 0.5x109/L