<p>Who we are at any given moment depends not only on our psychological dispositions but also the situational forces we experience. Here, we explore how stress – an intrapsychic situational force – impacts momentary personality expression. In Study 1 (<i>N</i> = 792), we experimentally induced stress, showing that the experience of stress is causally related to lower levels of state Openness, Conscientiousness, Extraversion, Agreeableness and higher levels of state Neuroticism. In Study 2 (<i>N</i> = 713; 17,853 observations), we used experience sampling to capture naturally occurring stress and show that stress is associated with lower levels of state Extraversion and Agreeableness, and higher levels of state Neuroticism and Openness in everyday situations. Notably, across both studies, the associations between stress and personality state expressions are largely robust when accounting for momentary affect. Together, our work highlights stress as a distinct and dynamic driver of personality state variability in everyday life.</p>