Objectives: We investigated the interactive effects of social support gap and daily stress on positive and negative affect among older adults. We hypothesized that higher levels of social support gap (i.e., undersupply) would be associated with lower levels of daily positive affect and higher levels of daily negative affect. We also hypothesized that there would be significant interactive effects of daily social support gap and daily stress on daily positive and negative affect. Methods: To test these hypotheses, we conducted a daily diary study with 145 older adults ranging from ages 65 to 89 (M = 73.5, SD = 6.0). Participants completed daily surveys assessing stress and how much emotional support they received and needed each day in addition to positive and negative affect. Results: As hypothesized, higher daily social support gap and higher daily stress were both associated with higher negative affect and lower positive affect. Additionally, daily social support gap moderated the association between daily stress and daily negative affect. A larger social support gap on days of high stress was related to higher daily negative affect compared to individuals who had a smaller social support gap. Discussion: These findings advance the current understanding of the stress-buffering effects of social support and highlight the importance of considering the gap between received and needed social support when considering the impact of social support on psychological well-being among older adults when they experience stress.