PURPOSE: Muscular strength is critical for adults with an intellectual disability (ID) to promote their mobility, cardiovascular capacity, and performance of daily living/recreational/vocational activities. This article reports the results of the first systematic review and meta-analysis of peer-reviewed clinical trials that evaluated the effects of resistance training (RT) interventions on muscular strength in adults with ID. METHODS: The protocol was registered with PROSPERO (CRD42020184905). The review focuses on clinical trials that recorded quantitative measures of maximum muscular strength. Eleven electronic databases were searched from their earliest available record up to May 2020. After screening 1996 search records, 11 clinical trials were reviewed. RESULTS: The RT interventions, while heterogeneous, had an overall significant (p ≤ 0.05) effect on muscular strength in adults with ID, ages 25-58 years. The findings were more significant and less heterogeneous for non-combined RT interventions than for interventions that combined RT exercises with aerobic or balance exercises. The TESTEX overall score was 8.3 ± 3.6. CONCLUSIONS: RT interventions (particularly when not combined with other exercises) are effective in promoting muscular strength in adults with ID. The limited number of studies and the low study quality scores indicate a potential risk of bias, which limits the interpretation of the findings and warrants further investigation.IMPLICATIONS FOR REHABILITATIONMuscular strength is critical for adults with an intellectual disability (ID) to promote their mobility, cardiovascular capacity, and performance of daily living/recreational/vocational activities.RT interventions are an effective means of improving muscular strength in adults with ID, especially when not combined with other forms of exercise.Testing and assessment protocols used in RT programs should be individualized for adults with ID to accommodate their characteristics and should be implemented under conditions similar to those experienced during the training regimen.It is important to implement familiarization sessions before carrying out muscular strength testing or initiating an RT program to ensure safety, accuracy, and effectiveness of the program for adults with ID.