Stress is a major health problem in today's workplace. Recent studies suggest that acceptance and commitment therapy (ACT) is effective for reducing stress at work, specifically among managers. Moreover, smartphone-delivered treatments have been developed and increasingly used in research. The objective of our study was to evaluate the efficacy of an ACT-based smartphone treatment among middle managers at medium- and large-sized companies (> 50 employees) in Sweden. A total of 73 participants were randomized to either receive the six-week stress intervention (n = 36) or to a waitlist control group (n = 37). Results showed small to moderate within-group effect sizes (Cohen's d range 0.37–0.62) for the treatment group, and small to moderate between group effects (Cohen's d range 0.41–0.50). In conclusion, the study indicates that a smartphone administered stress intervention based on ACT can reduce perceived stress and increase general health among Swedish middle managers in the private sector.