Understanding in vivo thumb muscle architecture is essential for advancing musculoskeletal modeling and identifying deviations linked to pathologies. As thumb muscle architecture has primarily been studied in cadavers, the objective of this study was to establish normative data on thumb muscle fascicle lengths in a young, healthy population using extended field-of-view ultrasound (EFOV-US). Six thumb muscles [abductor pollicis brevis (APB) and longus (APL);​ extensor pollicis brevis (EPB) and longus (EPL);​ flexor pollicis brevis (FPB) and longus (FPL)] and one wrist extensor [extensor carpi ulnaris (ECU;​ for comparison purposes only)] were imaged in 18 healthy adults (8 female;​ age:​ 22.7 ± 2.0 years;​ height:​ 172.1 ± 8.8 cm;​ weight:​ 79.0 ± 16.5 kg) [mean ± SD]. Measured fascicles were compared to cadaveric data (all thumb muscles) and ultrasound data (APB, ECU, FPL). Mean fascicle lengths (±SD) were 6.5 ± 0.8 cm (FPL), 3.8 ± 0.4 cm (APL), 4.7 ± 0.5 cm (EPL), 3.7 ± 0.5 cm (EPB), 4.5 ± 0.5 cm (APB), 3.6 ± 0.4 cm (FPB), and 4.2 ± 0.5 cm (ECU). The consistency of our measurements is indicated by the small standard deviations within (±0.1 to ± 0.7 cm) and across (±0.4 to ± 0.8 cm) participants. Measurement repeatability is high, as demonstrated by low coefficients of variation (range:​ 0.04–0.08) for the measured thumb muscles. We also examined to what extent anthropometric measurements can be used to predict fascicle lengths and found some significant relationships;​ however, these relationships were not consistent across all muscles. This study importantly expands our understanding of the complex anatomy of the healthy thumb and provides normative data for future work evaluating hand pathologies.