Background: Sibling relationships are very significant relationships, and their termination due to death will have profound long-term effects on the life of the surviving children. Objectives: The purpose of the current study was to determine the effectiveness of theraplay on internalizing and externalizing problems in bereaved siblings. Methods: This study was carried out using a single-subject multiple baseline design with follow-up. The statistical population included siblings and mothers of children aged 6 - 10 years who were on treatment at Mahak Pediatric Cancer Hospital and died one or two years ago. From this population, four mother-child pairs were selected as the study sample by convenience sampling. Each mother-child pair participated in 15 theraplay sessions once a week (45-min sessions). The assessments were carried out using the Child Behavior Checklist (CBCL) at baseline and in the intervention sessions and follow-up. Visual analysis of graphic displays of level, Reliable Change Index (RCI), and clinical significance were used to analyze the data. Data analysis and drawing the graphs were performed in Microsoft Excel 2016. Results: The results of the visual and quantitative data analysis showed a significant reduction in the four participants' internalizing problems during therapy. The reduction in the scores of this subscale in participants 1, 2, and 3 continued into the follow-up period and was noticeable. The results also showed that theraplay reduced the externalizing problems of most participants. A large proportion of these results persisted into the follow-up period. Conclusions: Theraplay appears to effectively reduce the internalizing and externalizing problems of bereaved siblings.