A Study to Assess Adverse Events and Change in Disease State of Intravenously (IV) Infused Etentamig (ABBV-383) of Adult Participants With Relapsed or Refractory Multiple Myeloma in Japan

Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) performance of \<= 2. * Must have adequate bone marrow function as defined in the protocol. * Must meet laboratory parameters as outlined in the protocol. * Must have a confirmed diagnosis of 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) criteria. * Relapsed defined as previously treated myeloma that progresses and requires initiation of salvage therapy, but does not meet criteria for refractory myeloma. * Refractory defined as disease that is nonresponsive (failure to achieve minimal response or development of progressive disease) while on primary or salvage therapy, or progresses within 60 days of last therapy. * Must have received at least 3 prior lines of therapy (including exposure to a proteasome inhibitor (PI), an immunomodulatory imide (IMiD), and an anti-CD38 mAb). * Must have measurable disease within 28 days of enrollment, defined as at least 1 of the following: * Serum M-protein \>= 0.5 g/dL (\>= 5 g/L). * Urine M-protein \>= 200 mg/24 hours. * Serum free light chain (FLC) \>= 100 mg/L (involved light chain) and an abnormal serum kappa lambda ratio only for participants without measurable serum or urine M-protein. * Consents to a fresh pretreatment bone marrow tumor biopsy or has adequate archival bone marrow tumor tissue that was collected within 12 weeks prior to screening and without intervening treatment. Exclusion Criteria: \- Has received B-cell maturation antigen (BCMA)-targeted therapy. Participants who have received targeted therapy against non-BCMA targets will not be excluded.