Many poststroke hemiplegic patients have an asymmetrical wheelchair-sitting posture. This study aimed to investigate the impact of different back support shapes on asymmetrical sitting posture and pressure points among poststroke hemiplegic patients during an activities of daily living-related reaching task. This study included 23 poststroke hemiplegic patients who performed tasks that involved the movement of objects using the unaffected upper limb to the affected side while sitting in a conventional wheelchair (C-WC) with a flat back support or a wheelchair providing pelvic and thoracic support (P-WC). Body alignment angles from video images and pressure distribution on supporting surfaces were measured using a two-dimensional motion analysis software (Dartfish) and a pressure mapping system (FSA). Regarding movement performance, although postural asymmetry increased in both wheelchair types, the degree of postural variation was smaller with P-WC use than C-WC use (p < 0.05), with partly reduced postural asymmetry. With P-WC, one-sided ischial asymmetrical pressure was significantly less after the movement (p < 0.05). In conclusion, P-WC's back support shape contributed to a decrease in postural asymmetry for pelvic girdle support both at rest and during movement. This highlights the importance of a wheelchair back support shape and may help to increase the quality of activities of daily living movement in poststroke hemiplegic patients in wheelchairs.