This is a Phase 2, open-label, single-arm, multicenter study to assess the antimyeloma activity and further characterize the safety, tolerability, PK, and PD of cemsidomide in combination with dexamethasone in participants with relapsed/refractory multiple myeloma (r/r MM).

Inclusion Criteria:

1. Be willing and able to provide signed informed consent for the study.
2. Age ≥ 18 years at the time of signed consent.
3. Subjects must have a documented diagnosis of MM and measurable disease at enrollment.
4. Received at least 3 prior anti-myeloma regimens (for a minimum of 2 or more consecutive cycles) that must have included an immunomodulatory drug (i.e., IKZF 1/3 degrader), a proteasome inhibitor, an anti-CD38 antibody, and a T-cell engager (TCE) or chimeric antigen receptor T-cell (CAR-T) therapy, unless not available per local standard of care.
5. Subjects need to have adequate organ function.
6. Toxicities from prior anticancer therapies must have resolved to baseline severity or CTCAE ≤ Grade 1.
7. Female subjects may not be pregnant or intend to become pregnant, may not breastfeed or intend to breastfeed, or donate ova during their participation in this study until 30 days after the last dose of study treatment.
8. Male subjects must agree to use a condom when having intercourse with a person of childbearing potential during the Treatment Period and for at least 30 days after the last dose of study treatment.
9. Male subjects must refrain from donating sperm during the Treatment Period and for 30 days after discontinuation.
10. Subjects must refrain from donating blood during study treatment and for 30 days after discontinuation.

Exclusion Criteria:

1. Presence of myeloma in the central nervous system (CNS).

2. Subjects with any of the following:

   • Systemic light chain amyloidosis
   • Polyneuropathy, Organomegaly, Endocrinopathy, Monoclonal gammopathy, and Skin changes (POEMS) Syndrome
   • Myelodysplastic syndrome (MDS).

3. Previously treated with cemsidomide.

4. Clinically significant impaired cardiac function or cardiac disease.

5. Thromboembolic event within 3 months prior to enrollment.

6. Known malignancy other than study indication that has progressed or required treatment within the past 3 years.

7. Uncontrolled active bacterial, fungal, or viral infection.

8. Inability or difficulty swallowing tablets.

NOTE: Other inclusion/exclusion criteria may apply, per protocol.