Background: The aim of this study was to systematically review the findings for validity, reliability, and acceptability of device-based measures of office-based physical activity and/or sedentary time in an office context to evaluate workplace interventions. Methods: The review was conducted according to the Preferred Reporting Items for Systematic Reviews and Analysis guidelines. Five electronic databases (PubMed, EMBASE, CINAHL, Cochrane, and Web of Science) were searched (inception to December 2023). Keywords included population (e.g., workers), type of measure (e.g., device-based), measurement constructs (e.g., validity), context (e.g., office), and behavior (e.g., sitting). Two authors screened titles, abstracts, and full texts independently with disagreements resolved by a third author. Findings were reported using narrative synthesis, and COnsensus-based standards for the Selection of health status Measurement INstruments was used for quality assessment. Results: In total, 2,299 articles were identified, with 16 articles retained. These reported 21 measurement protocols (nine in free-living settings) assessing eight worn, four remote, and one combined method. Sixteen protocols assessed office sitting, with standing (n = 8), moving (n = 11), postural transitions (n = 7), and location (n = 2) also assessed. Participant sample sizes ranged from one to 42 (median = 13). Criterion validity was assessed in all 21 protocols, with lower limb–worn measures of sitting, and worn and remote measures of location reporting the highest validity/accuracy compared with the ground truth (good to excellent). Only two articles reported acceptability (good acceptability), with none reporting reliability. Conclusions: There is evidence of valid device-based measures of office behavior (particularly sitting and location of workers), but this has largely been obtained in laboratory settings and/or with small samples. Larger studies in more varied free-living settings, potentially using multiples sources of data and assessing acceptability, are required.