This study examined whether participation in couples therapy, compared with individual therapy, had a differential effect on the day-to-day relationship between substance use and occurrences of intimate partner violence (IPV) among married or cohabiting substance-abusing men. Patients (N = 207) were randomly assigned to either partner-involved behavioral couples therapy (BCT; included non-substance-abusing female partners in conjoint sessions) or individual-based treatment (IBT; male partners only). Couples in BCT reported lower levels of IPV and substance use at a 12-month posttreatment follow-up compared with couples with male partners in IBT. Moreover, treatment assignment was a significant moderator of the day-to-day relationship between substance use and IPV. Likelihood of nonsevere and severe male-to-female partner violence on days of male partners' substance use was lower among couples who received BCT compared with IBT. These findings indicate couples therapy may play an important role in the treatment of IPV among substance-abusing couples.