Multiple Myeloma (MM) is a cancer of the blood's plasma cells ( blood cell). The cancer is typically found in the bones and bone marrow (the spongy tissue inside of the bones) and can cause bone pain, fractures, infections, weaker bones, and kidney failure. Treatments are available, but MM can come back (relapsed) or may not get better (refractory) with treatment. This is a study to determine adverse events and change in disease symptoms of etentamig (ABBV-383) in adult participants with relapsed/refractory (R/R) MM.

Etentamig (ABBV-383) is an investigational drug being developed for the treatment of R/R Multiple Myeloma (MM). This study is broken into 4 Arms; Arm A (Parts 1 and 2), Arm B and Arms C & D. Arm A includes 2 parts: step-up dose optimization (Part 1) and dose expansion (Part 2). In Part 1, different level of step-up doses are tested followed by the target dose of etentamig (ABBV-383). In Part 2, the step-up dose identified in Part 1 (Dose A) will be used followed by the target dose A of etentamig (ABBV-383). In Arm B a flat dose of etentamig (ABBV-383) will be tested. In Arms C & D, the step-up dose identified in Arm A will be used followed by the target dose of etentamig (ABBV-383) to investigate outpatient administration of etentamig (ABBV-383). Around 210 adult participants with relapsed/refractory multiple myeloma will be enrolled at approximately 50 sites across the world.

Participants will receive etentamig (ABBV-383) as an infusion into the vein in 28 day cycles for approximately 3 years.

There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and questionnaires.

Inclusion Criteria:

• Must have measurable disease as outlined in the protocol.
• Eastern Cooperative Oncology Group (ECOG) performance of <= 2. Arm C and Arm D: ECOG performance of <= 1.
• Relapsed/refractory (R/R) multiple myeloma (MM) with documented evidence of progression during or after the participant's last treatment regimen based on the investigator's determination of the International Myeloma Working Group (IMWG) 2016 criteria.
• Must be naïve to treatment with etentamig (ABBV-383).
• Arm A: Must have received at least 3 or more lines of therapy, including a proteasome inhibitor (PI), an immunomodulatory imide drug (IMiD), and an anti-CD38 monoclonal antibody.
• Arm B: Must have received at least 2 or more lines of therapy, including exposure to a PI, an IMiD, an anti-CD38 monoclonal antibody, and a prior B-cell maturation antigen (BCMA)-targeted therapy (must be an anti-drug conjugate [ADC] or chimeric antigen receptor T-cell [CAR-T] directed against BCMA).
• Arm C: Must have received at least 2 or more lines of therapy, including exposure to a PI, an IMiD, and an anti-CD38 monoclonal antibody. Must be suitable for outpatient administration of etentamig (ABBV-383).
• Arm D: Must have received at least 1 and no more than 3 prior lines of therapy, including exposure to a PI, an IMiD, or an anti-CD38 monoclonal antibody. Must be suitable for outpatient administration of etentamig (ABBV-383).

Exclusion Criteria:

• Arm A: Received BCMA-targeted therapy.
• Arm C and Arm D: Rapidly progressing disease per investigator.