The purpose of this study is to identify the recommended Phase 2 dose(s) (RP2Ds) and schedule assessed to be safe for Teclistamab and to characterize the safety and tolerability of Teclistamab at the RP2Ds.

The study will be conducted in 2 parts, separately for IV and SC administration: dose escalation (Part 1) and dose expansion (Part 2). It will evaluate safety, tolerability, pharmacokinetics and preliminary antitumor activity of Teclistamab administered to adult participants with relapsed or refractory multiple myeloma. The overall safety of the study drug will be assessed by physical examinations, Eastern Cooperative Oncology Group performance status, laboratory tests, vital signs, electrocardiograms, adverse event monitoring, and concomitant medication usage. Disease evaluations will include peripheral blood and bone marrow assessments at screening (performed within 28 days) and to confirm stringent complete response (sCR), complete response (CR), or relapse from CR. The end of study (study completion) is defined as 2 years after the last participant in Part 3 has received his or her initial dose of teclistamab. Study record NCT04557098 is Phase 2 part of this study and study record NCT03145181 is Phase 1 part of this study.

Inclusion Criteria:

• Documented diagnosis of multiple myeloma according to International Myeloma Working Group (IMWG) diagnostic criteria
• Measurable multiple myeloma that is relapsed or refractory to established therapies with known clinical benefit in relapsed/refractory multiple myeloma or be intolerant of those established multiple myeloma therapies, and a candidate for Teclistamab treatment in the opinion of the treating physician. Prior lines of therapy must include a proteasome inhibitor, an immunomodulatory drug and anti-CD38 monoclonal antibody in any order during the course of treatment. Participants who could not tolerate a proteasome inhibitor or immunomodulatory drugs and an anti-CD38 monoclonal antibody are allowed
• Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0 or 1
• Female participants of childbearing potential must use acceptable method of contraception
• Participants must sign an ICF indicating that he or she understands the purpose of and procedures required for the study and is willing to participate in the study. Consent is to be obtained prior to the initiation of any study-related tests or procedures that are not part of standard-of-care for the participant's disease

Exclusion Criteria:

• Prior treatment with any B cell maturation antigen (BCMA) targeted therapy
• Prior 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 multiple myeloma within 21 days; Cytotoxic therapy within 21 days; Proteasome inhibitor therapy within 14 days; Immunomodulatory agent therapy within 7 days; Gene modified adoptive cell therapy (example, chimeric antigen receptor modified T cells, natural killer [NK] cells) within 3 months; Radiotherapy within 14 days or focal radiation within 7 days
• Toxicities from previous anticancer therapies that have not resolved to baseline levels or to Grade 1 or less except for alopecia or peripheral neuropathy
• Received a cumulative dose of corticosteroids equivalent to >= 140 milligram (mg) of prednisone within the 14-day period before the first dose of study drug (does not include pretreatment medication)
• Known active central nervous system (CNS) involvement or exhibits clinical signs of meningeal involvement of multiple myeloma