<p>The effectiveness of the best-possible-self (BPS) intervention has been well established in Western people but is still under exploration in Eastern samples. A direct comparison of its effectiveness between Western and Eastern people is currently lacking, yet it is essential to comprehend the cultural sensitivity of the BPS. In this study, we compared immediate and short-term effects of the BPS between an Eastern (Chinese) and a Western (Dutch) sample using a three-time online intervention delivered within 1 week. We also investigated its effectiveness in both samples separately as well as the underlying mechanism. Participants (61 Chinese, 48 Dutch) were randomly assigned to the BPS or a control condition. We found that Dutch participants reported more benefits than the Chinese in terms of positive affect (immediate effect) and trait optimism as well as satisfaction with life (short-term effect). Chinese participants reported changes mostly in negative outcomes and only after multiple sessions. Moreover, the within-sample analysis showed that effect sizes were generally larger in the Dutch sample. The results replicate previous findings and add evidence supporting the effectiveness of the BPS in Dutch and Chinese people. This study indicates that BPS may be more effective in the Dutch sample for positive outcomes but can also be effective in the Chinese sample for certain negative outcomes with multiple sessions. Future studies may investigate underlying cultural sensitivity and adapt the BPS to a culture-compatible version based on specific cultural backgrounds.</p>