The purpose of this study is to determine the recommended phase 2 dose(s) (RP2D\[s\]) of JNJ-87562761 in Part 1 (dose escalation), and to determine the safety and tolerability at RP2D in Part 2 (dose expansion) in participants with multiple myeloma (MM) whose disease has come back after treatment (relapsed) or hasn't responded to treatment (refractory).