Objective: This study aimed to investigate the relationship between sleep habits and behavioral problems in preschool-aged children with ADHD. Materials and Methods: A cross-sectional study was conducted in a child developmental clinic at Siriraj Hospital between October and December 2023. Parents of children aged 4-6 years with ADHD completed the Children’s Sleep Habits Questionnaire (CSHQ) and the Strengths and Difficulties Questionnaire (SDQ). Descriptive analysis, Pearson correlation, and multivariate linear regression analysis were used to examine the association between sleep habits and behavioral problems. Results: This study recruited 80 preschoolers with ADHD and 75% were boys. The mean total CSHQ score was 43.76 ± 7.38 and 40% of the participants had sleep problems. 31.3 % had clinically significant behavioral problems. The most common behavioral problems were hyperactivity (33.8%), emotional problems (16.3%), and conduct behavior (10%). More sleep disturbances were highly correlated with all behavioral problems in SDQ, with a Pearson correlation between 0.78 and 0.90. After multivariate regression analysis, the total CSHQ score remained the strongest predictor of all domains of behavioral problems in preschool-aged children with ADHD (p < 0.05). Conclusion: More than 1/3 of preschool children with ADHD were affected by sleep disturbances. Furthermore, sleep problems in these children can contribute to significant behavioral problems. Therefore, the treatment of ADHD in preschoolers should always include the management of sleep disturbances.