In Switzerland, military service is a civic obligation for all adult male citizens, and thus, leadership style can be particularly challenging. The present study investigated the impact of superiors’ leadership styles on recruits’ achievement motivation, organizational citizenship behavior (OCB), and perceived stress during their Basic Military Training (BMT). To this end, a total of 525 male recruits (mean age: 20.3 years) recruits were assessed both cross-sectionally and longitudinally. At the start of BMT (baseline), at week 7, and at week 11, participants completed a series of self-rating questionnaires covering demographic information, achievement motivation, organizational citizenship behavior (OCB), perceived stress, and their superiors’ leadership styles (transformational, transactional und laissez-faire). Longitudinally, scores for achievement motivation and OCB showed no significant difference between baseline and the 11th week. In a group comparison, the group experiencing higher transformational leadership (from week 7 to week 11) had the highest scores for achievement motivation and OCB, and the lowest scores for perceived stress, all at week 11. Exploratively, achievement motivation and OCB at baseline were associated with transformational leadership and transactional leadership at week 7 and week 11. Perceived stress at baseline correlated only with transformational leadership but not with transactional leadership, both at week 7 and week 11. Transformational leadership style fostered achievement motivation and OCB in Swiss military recruits and protected them from stress, both cross-sectionally and longitudinally.