OBJECTIVE The Thumb Grasp and Pinch Assessment (T-GAP) is a new instrument for evaluating thumb use in children with congenital hypoplastic thumbs. The assessors video-record the children while they perform nine specific activities and score their grasp types using T-GAP. A high T-GAP score indicates more mature grasp patterns. The developers reported the instrument's validity and reliability for index finger pollicization. This study investigated T-GAP's validity and reliability in children with reconstructed hypoplastic thumbs. METHODS Four hand surgeons and two hand therapists from two hospitals rated video clips of 20 Manske type II and IIIa hands twice in 17 patients who performed the T-GAP at least 1 year after opposition transfer and thumb ligament reconstruction. To investigate the validity, we calculated correlation coefficients for T-GAP scores and clinical outcomes, including thumb ROM, grip and pinch strength, and visual analog assessments of thumb function and appearance. To estimate T-GAP's inter- and intrarater reliability, we calculated intraclass correlation coefficients and their 95% confidence intervals (CIs). RESULTS Thumb Grasp and Pinch Assessment score showed a strong linear correlation (r = 0.815-0.944) and a moderate to strong nonlinear correlation (ρ = 0.527-0.744) with visual analog scale assessments of thumb function and appearance, respectively; a moderate nonlinear correlation (ρ = 0.464) with grip strength; and a moderate nonlinear correlation (ρ = 0.541) with thumb MCP joint range of motion. The intraclass correlation coefficient for the interrater reliability was 0.892 (95% CI, 0.768-0.954) in round 1 and 0.898 (95% CI, 0.754-0.959) in round 2, and for intrarater reliability, the mean was 0.882 (95% CI, 0.785-0.980). CONCLUSIONS Thumb Grasp and Pinch Assessment score had a moderate to strong construct validity and a moderate concurrent validity. Both inter- and intrarater reliability was strong. CONCLUSIONS This study supports the T-GAP instrument's validity and reliability for assessing functional outcomes in congenital hypoplastic thumb reconstruction.