Y-27632, an inhibitor of the Rho-associated kinase ROCK, is a therapeutic lead for Huntington disease (HD). The downstream targets that mediate its inhibitory effects on huntingtin (Htt) aggregation and toxicity are unknown. We have identified profilin, a small actin-binding factor that also interacts with Htt, as a direct target of the ROCK1 isoform. Over-expression of profilin reduces aggregation of polyglutamine expanded Htt and androgen receptor (AR) peptides. This requires profilin's G-actin binding activity, and its direct interaction with Htt, which are both inhibited by ROCK1-mediated phosphorylation of profilin at Ser-137. Y-27632 blocks phosphorylation of profilin in HEK293 cells and primary neurons, which maintains profilin in an active state. Knockdown of profilin blocks the inhibitory effect of Y-27632 on both AR and Htt aggregation. A signaling pathway from ROCK1 to profilin thus controls polyglutamine protein aggregation, and is targeted by a promising therapeutic lead for HD.