Primary Objective: To evaluate the efficacy, safety, and tolerability of ABBV-383 (a BCMA x CD3 bispecific antibody) administered as monotherapy in adult subjects with RRMM who have received at least 2 prior lines of therapy, including exposure to a PI, an IMiD, and an anti-CD38 mAb. In the Phase 1 study (TNB383B.0001/M22-466), ABBV-383 monotherapy demonstrated encouraging efficacy with a manageable safety profile in subjects with RRMM treated with >= 3 prior lines of therapy with exposure to PI, IMiD and an anti-CD38 mAb. As of 17 May 2023, a total of 220 subjects have been treated with ABBV-383 administered intravenously. At the 60 mg Q4W dose level (n = 21), the ORR and >= VGPR rates were 65% and 50%, respectively. Secondary Objectives: Statistical testing of the secondary endpoints will be performed only if the primary analysis of PFS is statistically significant. The key secondary endpoints are: - OS - Rate of >= VGPR per IRC assessment - Rate of >= CR per IRC assessment - Rate of MRD negativity with >= CR, defined as achievement of CR or better by IMWG (2016) response criteria (per IRC assessment) and MRD negative status as assessed by NGS Adaptive Clonoseq at 10-5 threshold - Change from baseline to 6 months in disease symptoms as measured by the disease symptoms domain of the EORTC QLQ-MY20 - Change from baseline to 6 months in physical functioning as measured by the physical functioning domain of the EORTC QLQ-C30