Non-muscle myosin II (MyoII) contractility is central to the regulation of numerous cellular processes, including migration. Rho is a well-characterized modulator of actomyosin contractility, but the function of other GTPases, such as Rac, in regulating contractility is currently not well understood. Here, we show that activation of Rac by the guanine nucleotide exchange factor Asef2 (also known as SPATA13) impairs migration on type I collagen through a MyoII-dependent mechanism that enhances contractility. Knockdown of endogenous Rac or treatment of cells with a Rac-specific inhibitor decreases the amount of active MyoII, as determined by serine 19 (S19) phosphorylation, and negates the Asef2-promoted increase in contractility. Moreover, treatment of cells with blebbistatin, which inhibits MyoII activity, abolishes the Asef2-mediated effect on migration. In addition, Asef2 slows the turnover of adhesions in protrusive regions of cells by promoting large mature adhesions, which has been linked to actomyosin contractility, with increased amounts of active β1 integrin. Hence, our data reveal a new role for Rac activation, promoted by Asef2, in modulating actomyosin contractility, which is important for regulating cell migration and adhesion dynamics.