This Phase 3, 2-arm, randomized, active comparator-controlled, open-label, multicenter study will compare the efficacy and health-related quality of life (HR-QoL) and assess the safety of selinexor plus bortezomib (Velcade®) plus low-dose dexamethasone (SVd) versus bortezomib plus low-dose dexamethasone (Vd) in adult patients with RRMM who have received 1 to 3 prior anti-multiple myeloma (MM) regimens. Crossover from the Vd Arm to a treatment that includes selinexor (i.e., SVdX or SdX) will be allowed at the point of IRC-confirmed objective disease progression per the IMWG criteria for patients in the Vd Arm.