Multiple myeloma (MM) is a cancer of the blood's plasma cells. 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 the safety and pharmacokinetics of ABBV-383 in adult participants with relapsed/refractory (R/R) MM. ABBV-383 is an investigational drug being developed for the treatment of R/R MM. This study is broken into 3 Arms: Arm A with 2 parts and Arm B as an expansion. Participants will receive ABBV-383 as a subcutaneous (SC) injection and intravenous (IV) infusion in Arm A and SC injections of ABBV-383 in Arm B. Around 55 adult participants with relapsed/refractory multiple myeloma will be enrolled at approximately 25 sites across the world In Arm A participants will receive one of two doses of ABBV-383 as an SC injection and (IV) infusions, during the 151 week study duration. In Arm B, participants will receive the selected dose from Arm A as SC injections, during the 151 week study duration. 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.