Background: Active surveillance (AS) is a preferred management option for men with prostate cancer with favourable prognosis. However, nearly half of men on AS switch to treatment within 5 years, so therapeutic strategies to prevent or delay disease progression could be considered. The androgen receptor is the pre-eminent oncogenic driver in prostate cancer. Objective: To explore image-based tumour responses and the patient impact of short-duration androgen-targeted therapy (ATT) to abrogate disease progression during AS. Design setting and participants: Men on AS with Cambridge Prognostic Group 1 & 2 (low and favourable intermediate risk) prostate cancer and lesions visible on magnetic resonance imaging (MRI) were recruited to an open-label, single-centre, phase 2 feasibility study of short-term ATT (the TAPS01 study). Intervention: Apalutamide 240 mg was administered for 90 days. Outcome measurements and statistical analysis: MRI-measured tumour volume (TV), gland volume (GV), and the TV/GV ratio were calculated at baseline, at day 90 (end of treatment), and at 6- and 18-month follow-up. Quality of life metrics were measured at day 0, day 90, and 6 weeks after ATT. Results and limitations: Eleven patients (40% of eligible men approached) agreed to participate, of whom nine completed treatment. At day 90, the median percentage reduction was -38.2% (range -51.8% to -23.5%) for GV, -54.2% (range -74.1% to -13.8%) for TV, and -27.2% (range -61.5% to -7.5%) for TV/GV (all p < 0.0001). At 6 mo, while GV had returned to baseline (p = 0.95) both TV (-31.9%; p = 0.0007) and TV/GV (-28.7%; p = 0.0009) remained significantly reduced. This reduction was sustained at 18 months (TV -18%, TV/GV -23.8%; p = 0.01). European Organization for Research and Treatment of Cancer QoL core 30-item questionnaire scores for global, physical, role, and social functioning decreased during treatment, but all were recovering by 6 weeks. EQ-VAS scores were unchanged compared to baseline. Conclusions: TAPS01 has demonstrated feasibility and patient tolerability for short-term ATT in men on AS. Our data suggest a selective and durable antitumour effect in the short term and support a larger-scale randomised trial. Patient summary: We investigated the feasibility of short-term treatment with an androgen inhibitor to prevent or delay disease progression for men on active surveillance for prostate cancer. Results for a small group of patients show that 90-day treatment led to a sustained decrease in tumour volume over 18 months. The findings warrant a larger clinical trial for this approach, which could allow patients to delay or even avoid longer-term active treatments.