This study aimed to clarify whether an exercise involving weight shifting to the nonparetic side while standing on an inclined surface improves standing balance in the early phase after stroke. This assessor-blinded, randomized controlled trial included people undergoing inpatient rehabilitation at a university hospital. Participants (N = 52) with hemiparesis caused by a stroke were randomly assigned to an experimental group (n = 26) or control group (n = 26). Participants performed a weight-shifting exercise to the nonparetic side using a goal-directed reaching strategy while standing on an inclined surface that was elevated 5 degrees to the nonparetic side in the experimental group or a flat surface in the control group. The reaching exercise was conducted 30 times per day for 5days. Primary outcome was the Berg Balance Scale. Secondary outcomes were the posturographic examination (static standing and lateral weight shifting to the nonparetic and paretic sides), Trunk Control Test, Trunk Impairment Scale, Functional Ambulation Category, and Functional Independent Measure motor item scores. Through intention-to-treat analysis, no significant intervention effects were observed between groups on the Berg Balance Scale. A significant intervention effect was observed, however, with the experimental group on the lateral weight shifting to the nonparetic side in the mean percentage bodyweight values and center-of-pressure moving distance and to the paretic side in center-of-pressure moving distance and Functional Ambulation Category. There were no significant interaction effects concerning other outcomes. These results suggest that standing reaching exercises to the nonparetic side while standing on an inclined surface could improve lateral weight-shifting capacity and gait ability in participants in the early poststroke phase. This intervention should be incorporated into standard treatment programs focusing on the paretic side during early stroke rehabilitation.