Background: This study aimed to evaluate the impact of preoperative extra-articular proximal tibial varus deformities on preoperative and postoperative subjective outcomes in patients undergoing medial unicompartmental knee arthroplasty (UKA). Methods: A prospective study was performed among 104 patients undergoing robotic-arm assisted fixed-bearing medial UKA for isolated anteromedial compartment osteoarthritis. Patients were categorized based on their preoperative medial proximal tibial angle (MPTA), stratifying them into patients with tibial varus (MPTA <85°) and those without (MPTA ≥85°). Radiographic outcomes were assessed and patient-reported outcomes, including the UCLA activity score, the Oxford Knee Score, the Visual Analogue Scale for pain, Knee Injury and Osteoarthritis Outcome Score for Joint Replacement, and patient satisfaction were compared between groups preoperatively and at 12 and 24 months follow-up. Results: Postoperatively, patients with tibial varus exhibited more pronounced residual varus alignment (3.8° vs. 0.7°, P <0.001), as well as lower MPTA and joint line obliquity (JLO) values compared with patients without tibial varus. Patients with tibial varus deformities expressed significantly higher satisfaction levels following their surgery (93.4% vs. 77.6%, P = 0.026) and reported higher UCLA activity scores both at baseline (6.4 vs. 5.1, P = 0.006) and 1 year follow-up (7.6 vs. 6.6, P = 0.018) compared with those without tibial varus. However, at 24 months, no significant differences were observed in PROMs or clinical improvement in PROMs between groups. Conclusion: This study supports the safe utilization of fixed-bearing medial UKA as an effective surgical intervention for managing isolated anteromedial osteoarthritis, even in the presence of preoperative tibial varus deformities. While preoperative tibial varus may predispose patients to a more pronounced residual varus following medial UKA, subjective outcomes appear to remain unaffected.