In this study, we aimed to analyze the incremental effects of sensory integration therapy (SIT) plus neurodevelopmental therapy (NT) versus NT alone on the attention and motor skills in children with Down syndrome (DS). We randomly assigned into experimental (i.e. SIT + NT; n = 21) and control (i.e. NT alone; n = 21) groups. While NT was applied to both groups for six weeks, SIT was applied simultaneously to the experimental group for six weeks. Participants' motor functions (Bruininks-Oseretsky Test of Motor Proficiency--Short form (BOT-2 SF)) and attention skills (Stroop TBAG (Turkish Scientific and Technological Research Association) Form) were evaluated before and after treatment (6 weeks later). Stroop TBAG and BOT-2 SF scores of the groups were similar at the baseline (p > 0.05). Significant improvement from baseline was observed in both BOT-2 SF, and Stroop TBAG results in both groups (p < 0.05). In addition, the improvement in both BOT-2 SF and Stroop TBAG results was found to be greater in the experimental group compared to the control group (p < 0.01). There were clear advantages to adding SIT to NT alone when seeking to improve motor and attention skills in children with DS.