Medial-pivot total knee arthroplasty (TKA) is designed with high articular conformity in the medial compartment in order to achieve stability. The subjective outcome of patients has been reported to be good postoperatively; however, the association between the objective and subjective knee stability and the influence of subjective stability on the overall outcomes of TKA remained unknown. Our hypothesis was that postoperative flexion joint-gap imbalance could affect subjective knee instability following TKA in medial-pivot prostheses. The purpose of this study was to analyze the association between the joint gap in flexion and subjective knee instability and between subjective knee instability and outcomes following medial-pivot TKA. A total of 769 patients who underwent primary TKA with use of medial-pivot prostheses were enrolled. Clinical outcomes included knee range of motion, Knee Society Score-2011 (KSS-2011), Forgotten Joint Score-12 (FJS-12), patient-reported subjective knee instability, and axial knee radiography to assess flexion joint-gap balance measured at the final follow-up. Clinical outcomes were compared between patients with and without subjective knee instability. Moreover, associated factors were analyzed for postoperative subjective knee instability. Overall, 177 patients (23%) reported experiencing postoperative subjective knee instability. Knee flexion (p = 0.04); KSS-2011 symptom (p<0.001), satisfaction (p<0.001), expectation (p=0.008), and activity (p<0.001) subscales; and FJS-12 (p < 0.001) were significantly worse in patients with subjective knee instability. The KSS-2011 subjective score differences were greater than the minimal clinically important difference. Older age at the time of surgery (odds ratio, 1.04; p = 0.04) and a wider postoperative joint gap in flexion on the medial side (odds ratio, 1.21; p = 0.001) were significant risk factors for subjective knee instability. A gap angle of -2.9° (i.e., medial laxity) was the threshold to predict postoperative knee instability (sensitivity, 0.29; specificity, 0.91). Postoperative flexion joint-gap laxity on the medial side following medial-pivot TKA affected the patient-reported subjective knee instability scores. Postoperative KSS-2011 subjective scores following medial-pivot TKA were poorer in patients with subjective knee instability. Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.