The small G-protein Rap1 is a critical regulator of cell-cell contacts and is activated by the remodeling of adherens junctions. Here we identify the Rap1 guanine nucleotide exchange factor PDZ-GEF2 as an upstream activator of Rap1 required for the maturation of adherens junctions in the lung carcinoma cells A549. Knockdown of PDZ-GEF2 results in the persistence of adhesion zippers at cell-cell contacts. Activation of Rap1A rescues junction maturation in absence of PDZ-GEF2, demonstrating that Rap1A is downstream of PDZ-GEF2 in this process. Moreover, depletion of Rap1A, but not Rap1B, impairs adherens junction maturation. siRNA for PDZ-GEF2 also lowers the levels of E-cadherin, an effect that can be mimicked by Rap1B, but not Rap1A siRNA. Since junctions in Rap1B depleted cells have a mature appearance, these data suggest that PDZ-GEF2 activates Rap1A and Rap1B to perform different functions. Our results present the first direct evidence that PDZ-GEF2 plays a critical role in the maturation of adherens junctions.