Although the disengagement consequences of age-based stereotype threat in the workplace are well-documented, it is less clear what causes employees to experience age-based stereotype threat. Based on socioemotional selectivity theory, the present study examines whether and why daily cross-age interactions in the workplace lead to stereotype threat. Using a diary study design over 2 weeks, 192 employees (86 employees aged 30 and younger; 106 employees aged 50 and older), completed 3570 reports on daily interactions with coworkers. Results showed that both younger and older employees experienced stereotype threat when they engaged in cross-age interactions compared to interactions with people of a similar age. The characteristics of cross-age interactions that led employees to experience stereotype threat differed by age, however. Consistent with socioemotional selectivity theory, cross-age interactions were problematic for younger employees to the degree that they triggered concerns about competence, whereas concerns about warmth led to stereotype threat among older employees. Daily stereotype threat was associated with reduced feelings of workplace belonging for both younger and older employees but, contrary to expectations, stereotype threat was not related to energy and stress. These findings suggest that cross-age interactions can lead to stereotype threat for both younger and older employees, particularly when younger employees worry they are perceived as incompetent or older employees worry they are perceived as less warm. (PsycInfo Database Record (c) 2023 APA, all rights reserved).