Low back pain can be caused by prolonged sitting, originating in the sacroiliac joint (SIJ) in up to 30% of patients. The goal of this study was to develop a finite element model of the lower back and pelvis to study sitting configurations that could minimize the loads in the SIJ while sitting. The configurations were based on chair designs with geometries known to show some benefits according to literature: a 5° downward seat pan tilt and a 20° backrest recline. Both chairs were evaluated in neutral spine position with upright posture and 30° forward leaning configurations. A finite element model of the lumbar spine, pelvis and femurs was developed to compute the reaction forces at the SIJ. The intricate spinal geometry was simplified, and isotropic material properties were assumed for all components. The chair reaction forces were first computed analytically, then inputted as loads on the model. The results demonstrate that the improved sitting configuration reduced the loads in the SIJ compared to a conventional chair in both upright and forward leaning positions by 5.57 % body weight (BW) and 14.18 %BW, respectively. The proposed sitting configuration with a downward inclined seat pan and forward leaning back was shown to be an effective method to reduce SIJ loads.