The present study evaluated effects of dietary supplementation with tryptophan (Trp) on muscle growth, protein synthesis, and antioxidant capacity in hybrid catfish Pelteobagrus vachelli♀ × Leiocassis longirostris♂. Fish were fed six different diets containing 2.6 (control), 3.1, 3.7, 4.2, 4.7, and 5.6 g Trp kg−1 diet for 56 days, respectively. Results showed that dietary Trp significantly (1) improved muscle protein content, fiber density, and frequency of fiber diameter; (2) up-regulated the mRNA levels of PCNA, myf5, MyoD1, MyoG, MRF4, IGF-I, IGF-II, IGF-IR, PIK3Ca, TOR, 4EBP1, and S6K1; (3) increased phosphorylation levels of AKT, TOR, and S6K1; (4) decreased contents of MDA and PC, and increased activities of CAT, GST, GR, ASA, and AHR; (5) up-regulated mRNA levels of CuZnSOD, CAT, GST, GPx, GCLC, and Nrf2, and decreased Keap1 mRNA level; (6) increased nuclear Nrf2 protein level and the intranuclear antioxidant response element binding ability, and reduced Keap1 protein level. These results indicated that dietary Trp improved muscle growth, protein synthesis as well as antioxidant capacity, which might be partly related to myogenic regulatory factors (MRFs), IGFs/PIK3Ca/AKT/TOR, and Keap1/Nrf2 signaling pathways. Finally, based on the quadratic regression analysis of muscle protein and MDA contents, the optimal Trp requirements of hybrid catfish (21.82-39.64 g) were estimated to be 3.94 and 3.93 g Trp kg−1 diet (9.57 and 9.54 g kg−1 of dietary protein), respectively.