Inclusion Criteria:
* Capability of understanding the purpose of the study and have given written informed consent.
* Age greater than or equal to 18 years
* Histologically or cytologically confirmed relapsed or refractory MCL
* r/r MCL patients following standard first line chemotherapy who have received at least one prior regimen including ibrutinib
* If the participant has received prior bendamustine, response duration must have been \> 1 year
* Presence of at least one lymph node or mass measurable for response
* Life expectancy of at least 24 weeks
* ECOG 0-2
* Adequate hematological, renal and hepatic function unless inadequate function is due to underlying disease
Exclusion Criteria:
* History of severe allergic or anaphylactic reactions to humanized or murine monoclonal antibodies (MAbs or recombinant antibody-related fusion proteins) or known sensitivity or allergy to bendamustine or rituximab
* Contraindications to polatuzumab, bendamustine or rituximab
* Prior use of any MAb, radioimmunoconjugate, or antibody-drug conjugate (ADC) within 4 weeks or 5 half-lives before cycle 1 day 1
* Use of any investigational agent within 28 days prior to initiation of study treatment
* History of malignancy other than squamous cell carcinoma, basal cell carcinoma of the skin or carcinoma in situ of the cervix within the last 3 years
* Treatment with radiotherapy, chemotherapy, immunotherapy, immunosuppressive therapy, or any investigational agent for the purposes of treating cancer within 2 weeks prior to cycle 1 day
* Major surgery or significant traumatic injury within 28 days of the first dose of study drug
* Ongoing corticosteroid use \>30 mg per day prednisone or equivalent, for purposes other than lymphoma symptom control
* Autologous stem cell transplant (SCT) within 100 days prior to cycle 1 day 1
* Prior allogeneic SCT
* Eligibility for autologous SCT
* Primary or secondary CNS lymphoma
* Current grade \>1 peripheral neuropathy
* Evidence of significant, uncontrolled concomitant diseases that could affect compliance with the protocol or interpretation of results, including significant cardiovascular disease (such as New York Heart Association Class III or IV cardiac disease, myocardial infarction within the last 6 months, unstable arrhythmias, or unstable angina) or significant pulmonary disease (including obstructive pulmonary disease and history of bronchospasm)
* Known active bacterial, viral, fungal, mycobacterial, parasitic, or other infection (excluding fungal infections of nail beds) at study enrollment or any major episode of infection requiring treatment with IV antibiotics or hospitalization within 4 weeks prior to Cycle 1 Day 1
* Suspected or latent tuberculosis
* Positive test results for chronic hepatitis B virus (HBV) infection or for hepatitis C virus (HCV) antibody
* Known history of human immunodeficiency virus (HIV) seropositive status or known infection with human T-cell leukemia virus 1 (HTLV-1) virus
* Women who are pregnant or lactating or who intend to become pregnant within a year of the last dose of study treatment. Women of childbearing potential must have a negative pregnancy test at screening, pregnancy testing must be performed within 7 days before first administration of IMP. Approved methods of birth control must be used
* Women of childbearing potential, including women whose last menstrual period was less than one year prior to screening, unable or unwilling to use adequate contraception from study start to the last dose of protocol therapy. Adequate contraception defined as hormonal birth control, intrauterine device, double barrier method or total abstinence.
* Male subjects unable or unwilling to use adequate contraception methods.
* Evidence of laboratory abnormalities in standard renal, hepatic, or coagulation function tests
