Running is generally considered a symmetrical activity that involves harmonized functions of upper and lower limbs. However, asymmetry can occur under certain conditions, such as fatigue, as lower limbs perform distinct functional tasks. While running, trunk muscles play a crucial role in transmitting loads between the upper and lower limbs, yet the impact of trunk muscle fatigue on the dominant and nondominant legs has not been well addressed. This study investigated the effects of trunk muscle fatigue on ground reaction force characteristics on dominant and nondominant legs in novice runners. Thirty participants were asked to run along a runway at 3.3 m · s⁻¹ before and after a trunk muscle fatigue protocol. Ground reaction force data were collected bilaterally, and subsequent asymmetries were calculated. Trunk muscle fatigue had different effects on the dominant and nondominant legs. In the dominant leg peak medial force increased, while the nondominant leg showed reduced peak lateral force, peak braking force and peak negative free moment and increased medio-lateral impulse. Trunk muscle fatigue increased asymmetries in peak lateral force, peak braking force and decreased asymmetry in peak negative free moment. These findings suggest that trunk muscle fatigue, due to its different effects on dominant and nondominant legs, deteriorates running asymmetry and may increase injury risk in novice runners. Strengthening and improving the endurance of trunk muscles is recommended for novice runners to prevent strength reduction-related changes in running mechanics and mitigate injury risk.