This work adopts an Interdependence Theory framework to investigate how the features of interdependent situations that couples face in their daily life (i.e., situations in which partners influence each other’s outcomes) shape attachment security toward their current partners. An experience sampling study examined attachment tendencies and features of interdependent situations that people experience with their partner in daily life to predict satisfaction and trust in their relationship, and changes in attachment avoidance and anxiety toward their partner over time. Results revealed that encountering situations with corresponding outcomes (i.e., situations in which both partners have the same preferences) and with information certainty (i.e., situations in which there is clear knowledge of each partner’s preferences) assuage people’s insecurity. On the contrary, situations of mutual current and future interdependence (i.e., situations in which each person’s current or future outcomes are dependent on their partner’s behavior) undermined security for anxiously attached individuals. Power (i.e., the asymmetry in partners’ dependence) was not related to attachment security. This work underscores the importance of studying the role of the situations that partners experience in their daily life and the way they are related to relationship feelings and cognitions.