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Abstract Objective While polysubstance use is highly prevalent among people who use drugs, the field lacks a reliable assessment that can detect detailed temporal patterns of polysubstance use. This study assessed the test‐retest reliability of the newly developed Polysubstance Use–Temporal Patterns Section (PSU‐TPS). Methods Participants who used cocaine plus alcohol and/or marijuana at least once in the past 30 days (n = 48) were interviewed at baseline and approximately 7 days later (retest) using the Substance Abuse Module and the PSU‐TPS. Reliability of PSU‐TPS measures of quantity, frequency, and duration of polysubstance use was examined using intra‐class correlation coefficients (ICCs) and kappa tests. Results Excellent reliability was observed for frequencies of concurrent polysubstance use patterns in the past 30 days (ICC range: 0.90–0.94) and quantity of alcohol use (ICC = 0.83), and fair to good reliability was observed for duration of substance use (ICC range: 0.52–0.73). Conclusion Detailed information regarding cocaine, alcohol, and marijuana polysubstance use in the past 30 days can be reliably measured with the PSU‐TPS. Data on the order and timing of polysubstance use at the hourly level will improve our understanding of the implications of sequential and simultaneous use patterns, which can help inform treatment and prevention efforts.