Effective emotion regulation is critical for establishing and maintaining positive relationships, and it has previously been linked to several indicators of social competence. Theories agree that one core characteristic of adaptive emotion regulation is the ability to flexibly adapt emotion regulation strategies to situational demands (i.e., strategy flexibility). However, little is known about the development of this ability and its association with social competence at preschool age. We hypothesized a positive association of preschoolers' strategy flexibility with age, with overall regulation effectiveness as well as with different indicators of their social competence. To test these assumptions, we examined the emotion regulation strategies (i.e., distraction, gaze aversion, reappraisal, self-instruction, and self-soothing) of 60 preschoolers (M = 59.67 months; SD = 8.18; 45% girls) in two standardized situations, namely a frustration-eliciting persistence task and a challenging waiting situation, in which attentional deployment strategies have opposing effects, requiring flexible strategy use across situations. Children's social competencies in interaction with caregivers, peers, and task demands were assessed through teacher reports. Results reveal that the effectiveness of attentional deployment strategies changed as predicted. Furthermore, preschoolers' ability to adapt their emotion regulation strategies to the corresponding task demands was positively associated with age and overall regulation effectiveness. Concerning teacher-reported facets of children's social competence, strategy flexibility was significantly associated with children's integration into their peer group.