Forward head posture (FHP) induces forward shifts in the center of gravity (COG), affecting postural control and gait ability. Uphill and downhill walking require different postural control and gait strategies than overground walking due to the altered gravitational effect. The biomechanical characteristics of uphill and downhill walking in healthy adults have been demonstrated;​ however, those in individuals with FHP remain unclear. This study aimed to investigate COG, spatiotemporal, and kinematic parameters during uphill and downhill walking in individuals with FHP. This study included 10 individuals with FHP (FHP group) and 10 healthy adults (control group). All participants performed three experimental sessions:​ uphill, downhill, and overground walking. Spatiotemporal and kinematic parameters were measured using a motion analysis system. Results showed significant differences in COG parameters between groups in all sessions (p < 0.05), while no significant differences were found in spatiotemporal parameters (p > 0.05). In the uphill walking session, the mean and peak of head and knee joints during the stance phase in the FHP group were significantly greater than in the control group (p < 0.05). The FHP group showed a significantly greater mean of the knee joint during the stance phase than the control group in downhill and overground walking sessions (p < 0.05). Our results indicate that individuals with FHP have different gait mechanisms during uphill, downhill, and overground walking due to forward-shifted head COG relative to the body. These findings provide the biomechanical mechanisms related to uphill and downhill walking in individuals with FHP.