This work evaluated the effect of triiodothyronine (T3) on larviculture of matrinxã, Brycon amazonicus. Oocytes of three females were pooled, fertilized with pooled semen of two males and separated in four batches that were immersed in triiodothyronine solutions as follows: M1 (control – water); M2 (0.01 mg/L T3); M3 (0.05 mg/L T3); and M4 (0.1 mg/L T3). Triiodothyronine did not affect fertilization rate and number of hatched larvae. Weight of hatched larvae was significantly higher in treatments M3 and M4, as well as among larvae sampled at Day 12 in all treatments. After 12 d of rearing, biomass gain was higher in the hormone treatments (M1 688 ± 569 mg; M2 2436 ± 562 mg; M3 3572 ± 569 mg; and M4 4129 ± 770 mg). In general, coefficients of variation of weight (CVw) and length (CVl) did not differ among treatments and cannibalism was registered between 36 and 72 hours post-hatching (h.p.h.) without differences among treatments. Larval survival increased in the hormone treatments (M1 26.5%; M2 37.6%; M3 40.6%; and M4 40.8%). The results indicate that the immersion of matrinxã eggs in triiodothyronine can promote beneficial effects to its larviculture and indicate promising perspectives for culture of this tropical species.