ImportanceObservational data have shown that postdiagnosis exercise is associated with reduced risk of prostate cancer death. The feasibility and tumor biological activity of exercise therapy is not known.ObjectiveTo identify recommended phase 2 dose of exercise therapy for patients with prostate cancer.Design, Setting, and ParticipantsThis single-center, phase 1a dose-finding trial was conducted at a tertiary cancer center using a patientcentric, decentralized platform and included 53 inactive men with treatment-naive localized prostate cancer scheduled to undergo surgical resection between June 2019 and January 2023. Data were analyzed in June 2024.InterventionSix escalated exercise therapy dose levels ranging from 90 to 450 minutes per week of individualized, moderate-intensity treadmill walking, allocated using adaptive continual reassessment. All exercise therapy sessions were conducted remotely with real-time monitoring.Main Outcomes and MeasuresFeasibility was evaluated by relative exercise dose intensity (REDI). A dose level was considered feasible if 70% or more of patients achieved an REDI of 75% or greater. Activity end points were changes in tumor cell proliferation (Ki67) and plasma prostate-specific antigen levels between pretreatment and postintervention. Safety and changes in patient physiology were also assessed.ResultsA total of 53 men were enrolled (median [IQR] age, 61 [56-66] years). All dose levels were feasible (≥75% REDI). The mean (95% CI) changes in Ki67 were 5.0% (–4.3% to 14.0%) for 90 minutes per week, 2.4% (–1.3% to 6.2%) for 150 minutes per week, –1.3% (–5.8% to 3.3%) for 225 minutes per week, –0.2% (–4.0% to 3.7%) for 300 minutes per week, –2.6% (–9.2% to 4.1%) for 375 minutes per week, and 2.2% (−0.8% to 5.1%) for 450 minutes per week. Changes in prostate-specific antigen levels were 1.0 ng/mL (–1.8 to 3.8) for 90 minutes per week, 0.2 ng/mL (–1.1 to 1.5) for 150 minutes per week, –0.5 ng/mL (–1.2 to 0.3) for 225 minutes per week, –0.2 (–1.7 to 1.3) for 300 minutes per week, –0.7 ng/mL (–1.7 to 0.4) for 375 minutes per week, and –0.9 ng/mL (–2.4 to 0.7) for 450 minutes per week. No serious adverse events were observed. Overall, 225 minutes per week (approximately 5 minutes per treatment at 5 times weekly) was selected as the recommended phase 2 dose.Conclusions and RelevanceThe results of this nonrandomized clinical trial suggest that neoadjuvant exercise therapy is feasible and safe with promising activity in localized prostate cancer.Trial RegistrationClinicalTrials.gov Identifier: NCT03813615