The Rho family of guanosine triphosphatases (GTPases) function as binary molecular switches, which play an important role in the regulation of actin cytoskeleton rearrangement and are involved in several critical cellular processes including cell adhesion, division and migration. Rho GTPases are specifically activated by their associated guanine nucleotide exchange factors (RhoGEFs). Dysregulation of RhoGEFs function through mutation or overexpression has been implicated in oncogenic transformation of cells and linked to several kinds of invasive and metastatic forms of cancer. T-cell lymphoma invasion and metastasis 1 (Tiam1) is a multi-domain Dbl family GEF protein and specifically activates Rho GTPase Rac1 through the catalytic Dbl homology and Pleckstrin homology (DH-PH) bi-domain. Previous works have shown that the nucleotide exchange function of the full-length Tiam1 is auto-inhibited and can be activated by N-terminal truncation, phosphorylation and protein-protein interactions. However, the molecular mechanisms of Tiam1 GEF auto-inhibition and activation have not yet been determined. In this study, the N-terminal PH-CC-Ex domain of Tiam1 is shown to directly inhibit the GEF function of the catalytic DH-PH domain in vitro. Using fluorescencebased kinetics experiments, we demonstrate that the auto-inhibition of Tiam1 GEF function occurs by a competitive inhibition model. In this model, the maximum velocity of catalytic activity remains unchanged, but the Michaelis-Menten constant of the auto-inhibited Tiam1 (the PH-PH fragment) on the substrate Rac1 is increased compared to the activated Tiam1 (the catalytic DH-PH domain alone). Through small angle X-ray scattering (SAXS), the structure of auto-inhibited Tiam1 (the PH-PH fragment) is shown to form a closed conformation in which the catalytic DH-PH domain is blocked by the N-terminal PH-CC-Ex domain. Taken together, these findings demonstrate the molecular mechanism of Tiam1 GEF autoinhibition in which the PH-CC-Ex domain of Tiam1 inhibits its GEF function by preventing the substrate Rho GTPase Rac1 from accessing the catalytic DH-PH bi-domain.