Daytime variation affects the tolerance of cardiomyocytes to ischemia–reperfusion injury (IRI). This study aims to evaluate the impact of time-of-day reperfusion on clinical outcomes of remote ischemic conditioning (RIC) as an adjuvant to primary percutaneous coronary intervention(PPCI) in ST-elevation myocardial infarction(STEMI) patients. A post-hoc analysis of a prospective, single-center parallel 1:1 randomized trial (RIC-STEMI) was performed. This analysis included 448 STEMI patients previously randomized to either PPCI alone (PPCI group) (n= 217) or RIC as an adjuvant to PPCI (RIC + PPCI group) (n= 231). Moreover, the sample was divided according to the time of PPCI: night-morning (22 h-11h59min) (n= 216) or afternoon (12 h-21h59min) (n= 232) groups. The primary follow-up endpoint was a composite of cardiac death and hospitalization due to heart failure. There were no significant differences in the clinical characteristics and the follow-up outcomes between groups. The afternoon period (HR = 0.474; 95% CI 0.230–0.977;p= 0.043) and RIC (HR = 0.423; 95% CI 0.195–0.917;p= 0.029) were independent predictors of the primary follow-up endpoint. An univariate analysis showed a lower frequency of primary follow-up endpoint, just in the afternoon period (10.3%vs0.9%;p= 0.002), in the RIC + PPCI group. A multivariate analysis revealed that RIC was an independent predictor of the primary follow-up endpoint in the afternoon group (HR = 0.098; 95% CI 0.012–0.785;p= 0.029), but not in the night-morning group. In addition, the afternoon period was not an independent predictor of the primary follow-up endpoint when the multivariate analysis was performed in the PPCI group. In conclusion, this study showed an important cardioprotective effect of RIC, namely in the afternoon period, suggesting that the afternoon period enhances the cardioprotection induced by RIC.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00380-023-02247-8.