Cotton fiber development transition from elongation to secondary cell wall biosynthesis is a critical growth shifting phase that affects cotton fiber final length, strength and other properties. Morphological dynamic analysis indicates that an asynchronous fiber developmental pattern between two cotton species. The critical time point for Gh and Gb fiber elongation termination is, respectively, 23 and 27 days post-anthesis (dpa). The temporal changes of protein expression at three representative development periods (15–19, 19–23, 23–27 dpa) were examined in both species with iTRAQ technics. Strikingly, a large proportion of differentially expressed proteins (DEPs) was identified at 19–23 dpa in Gh or at 23–27 dpa in Gb, corresponding to their fiber developmental transition timing from elongation to secondary cell wall biosynthesis. To better understand fibers transitional development, we comparatively analyzed those DEPs in 19–23 dpa of Gh vs. in 23–27 dpa of Gb, and noted that these cotton species indeed share fundamentally similar fiber development features under the biological processes. It also showed that there have limited overlaps in both specific upregulated and downregulated proteins between the two species, suggesting specie-specific protein regulations in development. Proteomic profiling revealed dynamic changes of several key proteins and biological processes that potentially correlate with fiber development transition. During the transition, upregulated proteins mainly involved in carbohydrate/energy metabolism, oxidation-reduction, cytoskeleton, protein turnover, Ca²⁺ signaling etc, whereas important downregulated proteins mostly concentrated in phenylpropanoid and flavonoid secondary metabolism pathways. Several changed proteins in this key stage were also validated by qRT-PCR. Overall, the present study provides accurate pictures of the regulatory networks of functional proteins during the fiber developmental transition.