The purpose of this study is to evaluate the safety and tolerability of mRNA-2808 in participants with relapsed or refractory multiple myeloma (RRMM).

Key Inclusion Criteria:

• RRMM with prior exposure to a proteasome inhibitor, an immunomodulatory drug (IMiD), and an anti-cluster of differentiation (CD38) monoclonal antibody.

• Measurable disease defined as at least 1 of the following:

  • Serum M-protein ≥0.5 grams/deciliter
  • Urine M-protein ≥200 milligrams (mg)/24-hour
  • Involved free light chain (FLC) ≥100 mg/liter and an abnormal FLC ratio
  • Plasmacytoma with a single diameter ≥2 centimeters
  • Bone marrow plasma cells >30%

Key Exclusion Criteria:

• Known central nervous system (CNS) myeloma or clinical signs and symptoms of CNS involvement of myeloma.
• Active plasma cell leukemia, defined as peripheral blood plasma cells ≥20%.
• Radiotherapy or cytotoxic chemotherapy within 2 weeks prior to Day 1 (Baseline), except palliative radiotherapy of limited field is permissible within 2 weeks after discussion with the Sponsor medical monitor.
• Antibody-based immunotherapy (monoclonal antibody, bispecific antibody, antibody drug conjugate) within 21 days prior to Day 1 (Baseline).
• Proteasome inhibitor therapy or immunomodulatory agent within 14 days prior to Day 1 (Baseline).
• Autologous hematopoietic cell transplant within 100 days prior to Day 1 (Baseline).
• Allogeneic hematopoietic cell transplant within 180 days prior to Day 1 (Baseline).
• Genetically modified adoptive autologous or allogeneic cellular therapy (for example, chimeric antigen receptor T cell, chimeric antigen receptor natural killer) within 12 weeks prior to Day 1 (Baseline).
• Corticosteroid therapy ≥140 mg prednisone or equivalent cumulative dose within 14 days prior to Day 1 (Baseline).

Note: Other inclusion and exclusion criteria may apply.