Purpose: This study tested whether the Sitting Together and Reaching to Play (START-Play) physical therapy intervention indirectly impacts cognition through changes in perceptual-motor skills in infants with motor delays. Methods: Participants were 50 infants with motor delays randomly assigned to START-Play plus Usual Care Early Intervention (UC-EI) or UC-EI only. Infants' perceptual-motor and cognitive skills were assessed at baseline and 1.5, 3, 6, and 12 months post-baseline. Results: Short-term changes in sitting, fine motor skills, and motor-based problem-solving, but not reaching, predicted long-term changes in cognition. START-Play indirectly impacted cognition through motor-based problem-solving but not sitting, reaching, or fine motor skills. Conclusions: This study provided preliminary evidence that early physical therapy interventions that blend activities across developmental domains and are supported by an enriched social context can place infants on more optimal developmental trajectories. [This is the online version of an article published in "Pediatric Physical Therapy" (ISSN 0898-5669).]