The dynamic flow field in T-jet mixers is characterized by means of 3D computational fluid dynamics simulations. The main goal of this work is to identify and discuss the chaotic flow regime in T-jet reactors and the parameters that enable this flow regime, namely, the critical Reynolds number and design parameters of these mixers. The 3D chaotic flow regime that occurs in T-jet mixers is described herein with unprecedented detail regarding the mixing scales, flow dynamics, and coherent structures. The mixing scales are proven to be set by the mixer dimensions. The chaotic flow regime in T-jets is of practical interest due to its capability to promote very fast mixing, although descriptions in the literature are scarce.