BACKGROUND:Foot orthoses are a recommended treatment for patellofemoral (PF) pain and a number of lower limb osteoarthritic (OA) conditions. However, their mechanism of effect is poorly understood. RESEARCH QUESTION:To compare the immediate effects of foot orthoses and flat inserts on lower limb biomechanics, knee pain and confidence in individuals with PFOA. METHODS:Twenty-one participants (14 females; mean ± SD age 58 ± 8 years) with PFOA underwent three-dimensional motion analysis during level-walking, stair ascent, and stair descent under three footwear conditions: (i) their own shoes; (ii) prefabricated foot orthoses; and (iii) flat shoe inserts. Participants reported their average levels of knee pain and confidence after each task. Data were analysed with repeated-measures analysis of variance (ANOVA), effect sizes (partial eta squared), and Bonferroni post-hoc tests. RESULTS:During level-walking, there was a significant main effect of foot orthoses on peak ankle dorsiflexion angle (F2 = 0.773, p < 0.001, ƞ2 = 0.773) and peak ankle external dorsiflexion moment (F2 = 0.356, p = 0.046, ƞ2 = 0.356). Foot orthoses decreased the peak ankle dorsiflexion angle compared to the flat insert and shoe conditions, and decreased the peak ankle external dorsiflexion moment relative to flat inserts. During stair descent, there was a significant main effect of foot orthoses on peak ankle external dorsiflexion moment (F2 = 0.823, p = 0.006, ƞ2 = 0.738), with a trend towards lower peak dorsiflexion moment for foot orthoses compared to the flat insert and shoe conditions. No significant main effects were observed during stair ascent. No other lower limb biomechanical changes were observed across all three conditions. Knee pain and confidence scores were not significantly different across the three conditions. SIGNIFICANCE:Prefabricated foot orthoses altered sagittal plane biomechanics of the ankle during level-walking and stair descent in individuals with PFOA. Further research is required to determine whether these changes are clinically beneficial.