Objective: Self-care behaviors aimed at maintaining physical and mental health are often recommended during stressful contexts. We tested emotional predictors of self-care behaviors (healthy eating, exercise, engaging in a hobby, relaxation/meditation, time spent with a supportive person, talking online with friends/family) during the COVID-19 pandemic and their emotional consequences. We hypothesized a reciprocal within-person process whereby positive affect increases self-care behaviors (Hypothesis 1) and self-care behaviors increase positive affect while decreasing negative affect (Hypothesis 2). Method: A 10-day daily diary was completed by 289 adult participants in the United States during spring 2020 when counties in 40 out of 50 states had some form of stay-at-home orders. Results: Lagged analyses for Hypothesis 1 suggested that positive affect did not significantly predict residualized change in self-care behaviors; however, more intense negative affect predicted increased self-care behaviors from one day to the next. Concurrent analyses for Hypothesis 2 indicated most self-care behaviors were associated with more positive affect and some with less negative affect on the same day. Lagged analyses for Hypothesis 2 indicated that self-care behaviors largely did not predict residualized change in positive or negative affect from one day to the next. At the between-person level, people who experienced more positive affect engaged in more self-care behaviors across the sampling period. Conclusion: Self-care behaviors continue to have mental health benefits during stressful environments such as the COVID-19 pandemic and stay-at-home orders. Negative affect can play an adaptive role during times of stress by facilitating self-care. [This is the online version of an article published in "Health Psychology" (ISSN 0278-6133).]