To explore the benefits of a Partial Body Weight Support (PBWS) harness system within a play enriched environment on gross motor development and mastery motivation of infants with Down Syndrome (DS). A randomized crossover study with 17 pre-walking infants with DS in two conditions-play with or without the harness engaged-each for 3×/week over 3weeks with a 1-week washout. Assessments took place at baseline, crossover, and completion. Statistically and clinically significant changes were evident on the Gross Motor Function Measure-88; however, there were no significant changes in parent-reported mastery motivation. The combination of PBWS harness system support and high frequency-facilitated play within an enriched play environment positively affected gross motor development. The intervention did not impact mastery motivation skills, and the direct impact of the harness remains unclear.