Service members and veterans (SM/Vs) with posttraumatic stress disorder (PTSD) can receive significant benefits from social support by a spouse or romantic partner. However, little is known about how providing support impacts partners. This study sought to identify (a) how provision of support is associated with partners' daily negative and positive affect and (b) how SM/Vs' PTSD symptom severity might moderate such associations. In a 14-day daily-diary study that assessed 64 couples in which one member was an SM/V with PTSD symptoms, partners reported nightly on whether or not they provided instrumental support and/or emotional support that day as well as their current negative and positive affect. Multilevel modeling showed that the provision of emotional and instrumental support were both significantly related to partners' lower levels of negative affect, f 2 = 0.09, and higher levels of positive affect, f 2 = 0.03, on that same day but not the next day. The positive same-day effects were seen if any support was given, with no additive effects when both types of support were provided. Severity of SM/V PTSD moderated the association between provision of emotional support and lower same-day negative affect such that the association was significant only when PTSD symptoms were more severe. Overall, these findings indicate that support provision to a partner with PTSD is associated with improved affect for the romantic partner providing support. However, given that only same-day affect was associated with support, the findings may also suggest that positive affect increases the provision of support.