This study aims to compare clinical outcomes in unicompartmental knee arthroplasty (UKA) patients with or without magnetic resonance imaging (MRI) evidence of bone marrow edema (BME) in the patella and to evaluate the effect of functional outcomes after UKA in patients with patellofemoral osteoarthritis (PF OA). Outcomes of 146 knees in 141 patients who underwent medial UKA were included. According to their preoperative condition of patellofemoral joints, patients were divided into three groups: Group A, non-PF OA (Kellgren-Lawrence [K-L] scale = 0); group B, PF OA without BME (K-L ≥ 1, bone marrow edema pattern [BMEP] = 0); group C, PF OA with BME (K-L ≥ 1, BMEP ≥ 1). Clinical outcomes including visual analog scale (VAS) scores of knee pain, Hospital for Special Surgery (HSS) scores, and range of motions (ROMs) were evaluated and analyzed at the postoperative follow-up of 3 months and 2 years. From our results, BME was highly correlated to poor outcome in patients with UKA. At follow-up of 3 months, BME influenced the clinical outcome of UKA at an early postoperative stage in terms of VAS scores, HSS scores, and ROMs. At the final follow-up of 2 years, the clinical outcome was improved in terms of HSS score, although the anterior knee pain and active ROMs were still worse than that of patients without BME. In conclusion, there was no significant difference in clinical outcomes in patients without BME regardless of PF OA. However, the condition of BME should be taken serious consideration because of its indication of an adverse effect on the outcome after UKA.