Despite well-established evidence on marriage as a psychosocial support for adults, there are studies that indicate loneliness may affect even married adults. Loneliness provokes a dysregulation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Thus, the study aims to examine the sex-specific association of loneliness and cortisol levels in the married older population. A cross-sectional analysis was conducted among 500 married participants (316 male and 184 female) aged 65-90 years (mean age = 73.8 +/- 6.4 years) of the population-based KORA (Cooperative Health Research in the Region of Augsburg) - Age study. Linear regression analyses were employed to examine the association between cortisol measurements (salivary cortisol upon waking (M1), 30 min after awakening (M2), late night (LNSC), cortisol awakening response (CAR), diurnal cortisol slope (DCS)) and loneliness (assessed by UCLA Loneliness Scale) in married participants with adjustments for potential confounders. In total sample population, lonely married participants displayed a significantly flatter DCS after M2 peak than their not lonely counterparts. In sex-specific analyses, lonely married men showed flatter DCS and reduced CAR than non-lonely counterparts. The association between loneliness and DCS was robust even after adjustment for lifestyle and psychosocial factors. In married women, no significant associations between loneliness and cortisol levels were observed. These findings suggest a differential impact of loneliness on HPA axis dynamics in lonely married men. Our findings highlight the importance to address loneliness even in married people.