To create environments conducive to the wellbeing of workers, especially older workers, it is important to gain insight into ageism among young workers as a distinct segment in the workplace. However, this ageism remains unclear. This study explored the psychosocial predictors of discrimination against older Japanese workers by their younger male counterparts. Four models – Intergroup Contact Theory (ICT), Knowledge–Attitude–Behaviour Model (KABM), Terror Management Theory (TMT) and Frustration–Aggression Theory (FAT) – formed the basis of the study. All the models included positive and negative dimensions of stereotypes as mediators between predictors and discrimination. Data from 874 participants were obtained through a voluntary web survey for employees aged 25–39 years who lived in the Tokyo metropolitan area. A scale describing interactions with older workers (supportive, uncomfortable and avoidance), Facts on Ageing quiz, fears of being an older worker and a job dissatisfaction survey were employed to examine the hypotheses based on the above theories. The results supported the hypotheses based on ICT and FAT but not KABM. Higher supportive contacts were significantly related to lower discrimination mediated by a higher positive and lower negative dimension of stereotypes. Contacts characterised by higher discomfort were significantly related to higher discrimination mediated by a lower positive and higher negative dimension of stereotypes. Higher job dissatisfaction was related to higher discrimination mediated by lower positive stereotypes. Interestingly, higher fear of being an older worker was significantly related to lower discrimination mediated by higher positive stereotypes, contrary to the TMT-based hypotheses. Thus, ICT and FAT regarding ageism can explain young male workers’ discrimination against older workers in Japan, which differs culturally from Western countries. Furthermore, the results of the study suggest that boosting the quality of interactions as well as reducing bad interactions with older workers contribute to lower discrimination.