The main purpose of this study is to assess the safety of the combination of JNJ-63723283 and daratumumab (Part 1); to compare the overall response rate (ORR) in participants treated with JNJ-63723283 in combination with daratumumab versus daratumumab alone (Part 2); and to compare progression-free survival (PFS) in participants treated with JNJ-63723283 in combination with daratumumab versus daratumumab alone (Part 3).

This is a multi-phase study of JNJ-63723283 in combination with daratumumab compared with daratumumab alone in participants with multiple myeloma who have received at least 3 prior lines of therapy including a proteasome inhibitor (PI) and an immunomodulatory agent (IMiD) or whose disease is double refractory to both a PI and an IMiD. The study consists of Screening Phase (procedures performed within 28 days before enrollment), Open-Label Treatment Phase (with End-of-Treatment Visit to occur 4 weeks after the last dose of study treatment) and Follow-up phase (8 weeks after the last dose of study treatment). Ongoing safety evaluation during Part 1 and Part 2 will be overseen by the Safety Evaluation Team (SET). In Part 3, ongoing safety and efficacy evaluation will be performed by the Independent Data Monitoring Committee (IDMC).

Inclusion Criteria:

• Have received at least 3 prior lines of therapy including a proteasome inhibitor (PI) and an immunomodulatory agent (IMiD) in any order during the course of treatment for multiple myeloma or have disease that is refractory to both a PI and an IMiD
• Evidence of a response (partial response [PR] or better based on investigator's determination of response by International Myeloma Working Group [IMWG] criteria) to at least 1 prior treatment regimen
• Documented measurable disease for multiple myeloma at screening as defined in protocol
• Eastern Cooperative Oncology Group (ECOG) performance status score of 0, 1, or 2
• Contraceptive use by men or women should be consistent with local regulations regarding the use of contraceptive methods for participants participating in clinical studies

Exclusion Criteria:

• Received any of the following prescribed medications or therapies in the past: Anti-CD38 antibody, including daratumumab, and/or Anti-PD-1 (programmed death-1) and anti-PD-L1 (programmed death-ligand 1) antibodies
• Plans to undergo a stem cell transplant prior to progression of disease on this study (these participants should not be enrolled to reduce disease burden prior to transplant)
• History of malignancy (other than multiple myeloma) within 2 years prior to first administration of study drug (exceptions are squamous and basal cell carcinomas of the skin and carcinoma in situ of the cervix, or malignancy that in the opinion of the investigator, with concurrence with the sponsor's medical monitor, is considered cured with minimal risk of recurrence within 3 years)
• Clinical signs of meningeal involvement of multiple myeloma
• Known chronic obstructive pulmonary disease (COPD) with a forced expiratory volume in 1 second (FEV1) <50% of predicted normal or known moderate or severe persistent asthma within the past 2 years, or uncontrolled asthma of any classification

If an investigator wishes to publish information from the study, a copy of the manuscript must be provided to the sponsor for review at least 60 days before submission for publication or presentation. If requested by the sponsor in writing, the investigator will withhold such publication for up to an additional 60 days.