Background <p>This prospective observational study included patients undergoing primary total hip arthroplasty (THA). Shortened hospital stays after THA have increased the importance of patient self-management and early engagement in rehabilitation. Smartphone-based digital health applications are increasingly used in perioperative care; however, the clinical value of preoperative initiation remains unclear. To examine whether preoperative initiation of a smartphone-based physiotherapy support application improves postoperative patient satisfaction and behavioral outcomes compared with postoperative initiation in patients undergoing primary THA.</p> Methods <p>This prospective single-center observational study included patients who underwent primary THA between October 2024 and March 2025. Patients were classified into preoperative or postoperative application initiation groups based on the timing of use. The preoperative group started use at a mean of 19.3 ± 6.4 days before surgery (range: 7 to 30 days), while the postoperative group began use on postoperative day 1. Allocation was determined by smartphone ownership, patient preference, and timing of institutional implementation. Those who began application use one day before surgery were excluded. The primary outcome was the dissatisfaction score of the Japanese Orthopaedic Association Hip Disease Evaluation Questionnaire (JHEQ). Secondary outcomes included the Forgotten Joint Score–12 (FJS-12) and stages of behavior change. Outcomes were assessed preoperatively and at 1 week, 1, 3, and 6 months postoperatively.</p> Results <p>A total of 137 patients were analyzed. The preoperative initiation group demonstrated a lower JHEQ dissatisfaction score at 1 week postoperatively compared with the postoperative group (mean difference: -10.4, 95% confidence interval (CI): -19.62 to -1.20; unadjusted <i>p</i> = 0.03; adjusted <i>p</i> = 0.15 after Bonferroni correction). The effect size at 1 week was Cohen’s d = 0.39 (95% CI: 0.04 to 0.74). No significant between-group differences were observed at later time points or in FJS-12 scores. At 1 month, behavior change stage distributions differed between groups (<i>p</i> = 0.02).</p> Conclusions <p>Earlier (preoperative) initiation of a smartphone-based physiotherapy support application was associated with a clinically relevant reduction in patient dissatisfaction at one week postoperatively and more favorable early behavioral readiness for rehabilitation. Shifting the timing of digital health support to the preoperative phase, thereby facilitating earlier exposure to education and guidance, may enhance the early postoperative patient experience and facilitate engagement. The clinical magnitude of this effect was supported by the effect size and confidence interval at the initial recovery stage.</p>