Multiple myeloma (MM) is an incurable disease characterized by the growth of monoclonal plasma cells in the bone marrow. The purpose of this study is to assess the adverse events and change in disease state of etentamig in adult participants with relapsed/refractory (R/R) multiple myeloma (MM). Adverse events and change in disease state will be assessed. Etentamig (ABBV-383) is an investigational drug being developed for the treatment of R/R MM. Study doctors put the participants in groups called treatment arms. Two doses of ABBV-383 will be explored. Each treatment arm receives a different dose of ABBV-383 to determine a tolerable dose. Approximately 12 adult participants with R/R MM will be enrolled in the study in approximately 6 sites in Japan. Participants will receive intravenous (IV) Etentamig (ABBV-383) at two increasing doses in 21-day cycles. 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 an approved institution (hospital or clinic). The effect of the treatment will be frequently checked by medical assessments, blood tests, and and monitoring of side effects.