Cotton (Gossypium spp.) is an important economic crop and the largest source of textile fiber in the world. However, to date, only a few genes have been identified that exhibit critical roles in fiber development, and few has shown positive effects on fiber yield and quality in transgenic cotton. Here, we report the characterization of a novel sucrose synthase (SusA1) gene from a superior quality fiber germplasm line 7235 in Gossypium hirsutum. By association analysis, GhSusA1 was highly correlated with fiber qualities in (7235× TM-1) recombinant inbred lines based on polymorphism of GhSusA1 between 7235 and TM-1. Subsequently, based on an interspecific population of 141 BC1 individuals generated from the cross between TM-1 and Gossypium barbadense line, Hai7124, we further mapped GhSusA1 genes on homeologous chromosomes A8 (chro.8) and D8 (chro.24). Suppression of GhSusA1 in transgenic cotton reduced fiber quality and decreased the boll size and seed weight. Importantly, overexpression of this gene increased fiber length and strength, with the latter indicated by the enhanced thickening of cell wall during secondary wall formation stage. Moreover, increasing GhSusA1 transcript abundance in vegetative tissues led to elevated seedling biomass. Together, these findings identified GhSusA1 as a key regulator of sink strength in cotton, which is tightly associated with productivity, and hence a promising candidate gene that can be developed to increase cotton fiber yield and quality.