Inclusion Criteria:

* Be ≥18 years of age (or the higher legal age in the jurisdiction in which the study is taking place) at the time of informed consent
* Has documented diagnosis of MM according to the IMWG diagnostic criteria (Rajkumar 2011).
* Has received 4 or more prior MM therapies including a PI, IMiD and CD38 antibody.
* Has an ECOG performance status (Oken 1982) of 0 to 1.
* Adequate organ system function
* Body weight \>35 kg.
* A participant of childbearing potential must have a negative highly sensitive serum (β-hCG) at screening and within 72 hours of the start of study treatment and must agree to further serum or urine pregnancy tests during the study.
* A participant must agree not to be pregnant, breastfeeding, or planning to become pregnant while enrolled in this study or within 5 months after the last dose of study treatment.
* A participant must be:
* A- Not of childbearing potential. If a participant becomes of childbearing potential after start of the study the participant must comply with point (b) as described below.

b-Of childbearing potential and Practicing at least 1 highly effective method of contraception (failure rate of \<1% per year when used consistently and correctly) and agrees to remain on a highly effective method while receiving study treatment and until 3 months after last dose - the end of relevant systemic exposure. Participant must agree to continue the above throughout the study and for 5 months after the last dose of study treatment.
* A participant must agree not to donate gametes (ie., eggs or sperm) or freeze for future use for the purposes of assisted reproduction during the study and for a period of 5 months after receiving the last dose of study treatment. Participants should consider preservation of gametes prior to study treatment as anti-cancer treatments may impair fertility.
* A 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 a period of 3 months after receiving the last dose of study treatment. A partner of childbearing potential must also be practicing a highly effective method of contraception.
* A participant must sign an ICF indicating that participant understands the purpose of, and procedures required for, the study and is willing to participate in the study.
* A participant is required to stay within 30 minutes of transportation to the site and remain in the company of a competent adult at all times until 48 hours following administration of all doses within the teclistamab step-up dosing schedule (step-up doses 1 and 2 and the first treatment dose) (approximately 10 days).
* A participant must agree to carry the study participant identification wallet card at all times.
* A participant must comply with all the protocol requirement procedures, including measuring and recording of body temperature and blood oxygen saturation twice daily (≥8 hours apart) during the first 2 cycles of teclistamab treatment and coming to the study site for safety assessments.
* A participant and the accompanying competent adult must be made aware of the presenting sign sand symptoms of teclistamab-associated toxicities, including but not limited to CRS, ICANS, infections, etc. The accompanying competent adult must watch the participant at all times for teclistamab-associated toxicities, until 48 hours after the first treatment dose of teclistamab

Exclusion Criteria:

* Has high tumor burden, defined as having ≥60% plasma cell infiltrate on the bone marrow biopsy or aspirate, whichever is higher, or with multiple extramedullary disease sites or plasmacytomas.
* Has a rapidly progressing disease per investigator assessment.
* Has plasma cell leukemia (\>2.0×10\^9/L plasma cells by standard differential), Waldenström's macroglobulinemia, POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes), or primary amyloid light-chain amyloidosis.
* Has known active CNS involvement or exhibits clinical signs of meningeal involvement of MM.
* Has risk factors for developing clinically significant TLS and requiring management with increased hydration, allopurinol, or rasburicase.
* Has myelodysplastic syndrome or active malignancies (ie, progressing or requiring treatment change in the last 12 months) other than RRMM. The only allowed exceptions are:

Any malignancy that was not progressing nor requiring treatment change in the last 12 months.

Malignancies treated within the last 12 months and considered at very low risk for recurrence:

Non-muscle invasive bladder cancer (solitary Ta-PUNLMP or low grade, \<3 cm, no CIS).

Skin cancer (non-melanoma or melanoma). Noninvasive cervical cancer. Breast cancer: adequately treated lobular carcinoma in situ or ductal carcinoma in situ, localized breast cancer and receiving antihormonal agents.

Localized prostate cancer (M0, N0) with a Gleason Score ≤7a, treated locally only (RP/RT/focal treatment).

Other malignancy that is considered at minimal risk of recurrence.

* Has Grade ≥3 hematologic AEs or Grade ≥3, clinically significant non-hematologic AEs.
* Has fever or active infection (bacterial, viral, or uncontrolled systemic fungal) at time of study enrollment.
* Has active autoimmune disease or a documented history of autoimmune disease with the exception of vitiligo, type I diabetes, and prior autoimmune thyroiditis that is currently euthyroid based on clinical symptoms and laboratory testing.
* Has clinically significant coagulopathy that would increase the risk of bleeding in the setting of cytopenia.
* Shows a deterioration in neurologic status, including mental status changes such as confusion or increased somnolence.
* Has psychiatric disorders (eg, alcohol or drug abuse), dementia, or altered mental status that would compromise the ability to provide informed consent or comply with the clinical protocol.
* History of stroke or seizure within 6 months of signing ICF.
* Presence of the following cardiac conditions:

New York Heart Association stage III or IV congestive heart failure. Myocardial infarction or CABG ≤6 months prior to enrollment. History of clinically significant ventricular arrhythmia or unexplained syncope, not believed to be vasovagal in nature or due to dehydration.

History of severe non-ischemic cardiomyopathy. Poorly controlled coronary artery disease and/or congestive heart failure. Uncontrolled cardiac arrhythmia or clinically significant ECG abnormalities.

* Has hepatitis B infection (ie, HBsAg or HBV-DNA positive). In the event the infection status is unclear, quantitative viral levels are necessary to determine the infection status.
* Has active hepatitis C infection as measured by positive HCV-RNA testing. Participants with a history of HCV antibody positivity must undergo HCV-RNA testing. If a participant with history of chronic hepatitis C infection (defined as both HCV antibody and HCV-RNA positive) completed antiviral therapy and has undetectable HCV-RNA 12 weeks following the completion of therapy, the participant is eligible for the study.
* Has COPD with FEV1 \<50% of predicted.
* Has eGFR \<20 ml/min or is dependent on dialysis.
* Has other medical issue that would impair the ability of the participant to receive or tolerate the planned treatment at the investigational site, to understand informed consent or any condition for which, in the opinion of the investigator, participation would not be in the best interest of the participant (eg, compromise the well-being) or that could prevent, limit, or confound the protocol-specified assessments.
* Has received packed RBC or platelet transfusions within the last 7 days prior to dosing.
* Has contraindications to the use of tocilizumab or IVIG per local prescribing information.
* Has received live vaccine(s) within 1 month prior to screening or plans to receive live vaccines during the study.
* Has received live, attenuated vaccine within 30 days before the first dose of teclistamab. Live, attenuated influenza vaccines are permitted as late as 30 days before the study treatment.
* Has received an investigational intervention or used an invasive investigational medical device within 14 days before the planned first dose of study treatment or received an investigational biological product within 14 days or 5 half-lives, whichever is longer, before the planned study treatment, or is currently enrolled in an investigational study.
* History of antitumor therapy as follows, before the first dose of study drug:

Targeted therapy, epigenetic therapy, or treatment with an investigational drug or used an invasive investigational medical device within 21 days or at least 5 half-lives, whichever is less.

Monoclonal antibody treatment for MM within 21 days. Cytotoxic therapy within 21 days. PI therapy within 14 days. Immunomodulatory agent therapy within 7 days. Gene modified adoptive cell therapy (eg, chimeric antigen receptor modified T cells, NK cells) within 3 months.

Radiotherapy within 14 days or focal radiation within 7 days. Prior CAR-T or bispecific antibody therapy.

•History of stem cell transplant:

1. An allogeneic stem cell transplant within 6 months. Participants who received an allogeneic transplant must be off all immunosuppressive medications for ≥42 days without signs of graft-versus-host disease.
2. An autologous stem cell transplant ≤12 weeks before the first dose of study drug.