This is a Phase 2, single-center study to explore the efficacy, safety, and tolerability of up to two 25-mg doses of psilocybin administered at an interval of 9 to 10 weeks in patients with MDD and cancer. This two-part study will administer a fixed dose (25 mg) of psilocybin in a double-blind, randomized, placebo-controlled portion (Dosing Session 1) and subsequently allow rollover into an open-label portion (Dosing Session 2; fixed dose of psilocybin, 25 mg) for patients who do not achieve remission of MDD symptoms after the first dose. In Dosing Session 1, groups of two to four patients will be randomized, as a cohort, to receive either psilocybin 25 mg or niacin 100 mg (active placebo) in a group session, with each patient supported by their dedicated study therapist and monitored by a second therapist via video feed. In Dosing Session 2, all eligible participants (i.e., patients who have not achieved remission defined as MADRS \< 10 at V7) will receive psilocybin 25 mg in an open-label fashion using the group session model. The study population will include adult men and women who are 18 years of age or older and have diagnoses of both MDD and a malignant neoplasm. MDD is defined as the Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) diagnostic criteria for a single or recurrent episode of MDD without psychotic features. A diagnosis of a malignant neoplasm is defined as having a diagnostic code from C00 to C97 according to the International Classification of Diseases, 10th edition (ICD-10). Participants will be recruited through referrals from specialized psychiatric and oncology services as well as through patient self-referrals. The majority of participants will have no prior exposure to psilocybin or so-called magic mushrooms; however, participants with prior recreational experience with psilocybin or magic mushrooms are eligible.