OBJECTIVES: Foot orthoses are commonly used in the management of knee OA, although the relationship between foot function and knee OA is still unclear. The purpose of the study was to examine foot function during walking in people with and without medial compartment knee OA. METHODS: Motion of the tibia, rearfoot and forefoot in 32 patients with medial compartment knee OA and 28 age-matched control subjects was investigated. Multivariate analysis was used to compare the groups. RESULTS: The knee OA group contacted the ground with a more everted rearfoot, demonstrated greater peak rearfoot eversion and exhibited reduced rearfoot frontal plane range of motion and reduced rearfoot peak inversion. The tibia was more internally rotated and laterally tilted throughout the gait cycle, with reduced peak external rotation. CONCLUSION: People with medial compartment knee OA exhibit altered foot kinematics during gait that are indicative of a less mobile, more everted foot type. The presence and degree of tibial malalignment and the available rearfoot range of motion during walking may affect individual responses to load-altering interventions, such as foot orthoses and footwear modifications. TRIAL REGISTRATION: Australian New Zealand Clinical Trials Registry, www.anzctr.org.au/, ACTRN12608000116325.