Abstract Purpose: This phase I, dose-escalation study investigated the recommended dose for expansion (RDE) of siremadlin, a p53–MDM2 inhibitor, in patients with wild-type TP53 advanced solid or hematologic cancers. Patients and Methods: Initial dosing regimens were: 1A (day 1; 21-day cycle; dose 12.5–350 mg) and 2A (days 1–14; 28-day cycle; dose 1–20 mg). Alternative regimens included 1B (days 1 and 8; 28-day cycle) and 2C (days 1–7; 28-day cycle). The primary endpoint was incidence of dose-limiting toxicities (DLT) during cycle 1. Results: Overall, 115 patients with solid tumors and 93 with hematologic malignancies received treatment. DLTs occurred in 8/92 patients with solid tumors and 10/53 patients with hematologic malignancies. In solid tumors, an RDE of 120 mg was defined in 1B. In hematologic tumors, RDEs were defined in 1A: 250 mg, 1B: 120 mg, and 2C: 45 mg. More patients with hematologic malignancies compared with solid tumors experienced grade 3/4 treatment-related adverse events (71% vs. 45%), most commonly resulting from myelosuppression. These were more frequent and severe in patients with hematologic malignancies; 22 patients exhibited tumor lysis syndrome. Overall response rates at the RDEs were 10.3% [95% confidence interval (CI), 2.2–27.4] in solid tumors and 4.2% (95% CI, 0.1–21.1), 20% (95% CI, 4.3–48.1), and 22.2% (95% CI, 8.6–42.3) in acute myeloid leukemia (AML) in 1B, 1A, and 2C, respectively. Conclusions: A common safety profile was identified and preliminary activity was noted, particularly in AML. Comprehensive investigation of dosing regimens yielded recommended doses/regimens for future combination studies.