The Russian aggression of Ukraine has put millions of civilians under immense stress and forced many of them to leave their homes for safety and help. Poland became one of the leading destinations for waves of Ukrainians fleeing this war. The level of perceived stress in people who experienced war depends on various factors, including individual psychological variables. The main aim of this study was to analyze perceived stress levels and the predictive role of affective temperaments, as defined by Akiskal, for perceived stress in Ukrainians and Poles during the first year of the Russo-Ukrainian war. Secondly, we studied the relationship between affective temperament, stress, and commitment to help Ukrainian refugees. Self-report data from 410 Ukrainians and 146 Poles were collected. The results of this study shed light on perceived stress in war-affected populations and the role of affective temperaments in predicting its levels. We also demonstrated the links between affective temperament, perceived stress, and involvement in helping Ukrainian refugees. Understanding the mental status of people affected by war and its predictors is crucial to providing appropriate support and assistance to those in need.