Flexible use of emotion regulation (ER) strategies in daily life is theorized to depend on appraisals of occurring stressful events. Yet, to date, little is known about (a) how appraisals of the current situation modulate the use of ER strategies in daily life and (b) how individual differences in affective symptoms impact these relations among appraisals and ER strategy use. This study attempted to address these two limitations using a 5-day experience sampling protocol, with three surveys administered per day in a sample of 97 participants. Each survey measured momentary appraisals of stress intensity and controllability as well as ER strategy use (i.e., rumination, reappraisal, avoidance, and active coping). Results showed that, in situations of low-stress intensity, higher stress controllability was related to greater use of reappraisal and rumination. In situations of high-stress intensity, higher controllability was related to reduced use of rumination. This pattern of flexible use of ER strategies depending on momentary stress appraisals was found for both rumination and avoidance and occurred specifically in individuals reporting lower levels of depression and/or anxiety levels. These findings provide new insight into how flexible use of ER strategies in daily life is modulated by interactions between stress intensity and controllability appraisals at varying levels of affective symptoms.