Does the overtness of external support to rebels affect civilian targeting in civil wars? Conflict studies increasingly scrutinize how insurgent sponsorships shape rebels’ behavior. However, the influence of external sponsors’ decisions to publicly acknowledge or deny their support on rebel conduct is largely neglected. This article introduces a new dataset on the overtness of external support to rebels in civil wars between 1989 and 2018. It then assesses whether the overtness of support is correlated with insurgents’ propensity to target civilians. I hypothesize that overtly supported rebels are less likely to target civilians than covertly supported rebels. This hypothesis stems from how supply-side factors—the way state sponsors expectedly act after having allocated their support—impact insurgents’ structure of incentives around relations with non-combatants. Statistical analyses yield strong support for my hypothesis. Moreover, further analyses show that support overtness influences civilian targeting independently from sponsors’ characteristics, such as political regimes or foreign aid reliance. Thus, in addition to the type of material aid insurgents receive, variation in whether support is covert or overt shapes how rebels treat civilians.