|Abstract This study investigated the tibiofemoral joint (TFJ) forces and supporting muscle forces displayed by knee osteoarthritis (OA) patients during stair ascent, and if these forces were influenced by the presence of pain. Fifteen knee OA patients partitioned into two groups based on pain experienced during stair ascent trails using a Visual Analogue Scale (VAS) (OA‐pain = 10; OA‐no pain = 5) and 14 healthy aged‐matched controls took part in this study. Kinematic and kinetic data were collected during three stair ascent trials, which provided the inputs for the musculoskeletal model FreeBody. TFJ contact forces and muscles forces were predicted by the model at early, mid‐ and late stance. These variables were compared between groups using a one‐way analysis of variance. The results show the OA‐pain (P < .05; d = 2.4) and OA‐no pain (P < .05; d = 1.1) groups displayed reduced medial TFJ contact forces and altered muscle forces in comparison to healthy controls during early stance. This suggests pain and the anticipation of pain results in knee OA patients deliberately offloading the front limb during stair ascent, which alters supporting muscle forces. This study provides valuable information on knee OA mechanics during stair ascent for clinicians developing rehabilitation programs.