Abscisic acid (ABA) is an important plant hormone that modulates seed germination and plant growth and stress responses, but its signaling remains poorly understood. We investigated the role of ROP10, a member of the Arabidopsis Rop subfamily of Rho GTPases, in ABA signaling. A null rop10 mutant exhibits enhanced responses to ABA in seed germination, root elongation, and stomatal closure assays and in the induction of expression of the transcription factor MYB2, but it shows wild-type levels of ABA and normal responses to other hormones. Consistently, transgenic expression of a constitutively active form of ROP10 reduces ABA inhibition of seed germination, whereas dominant-negative mutants of ROP10 enhance ABA response and partially suppress abi2. Furthermore, ABA specifically downregulates ROP10 transcription in root tips. ROP10 is localized to the plasma membrane (PM), and PM localization is crucial for its function. These results suggest that ROP10 is a PM-localized signaling molecule that is involved specifically in the negative regulation of ABA signaling.