Objectives The aim of this study was to examine the level of agreement between acute instantaneous wave-free ratio (iFR) measured across nonculprit stenoses in patients with ST-segment elevation myocardial infarction (STEMI) and iFR measured at a staged follow-up procedure. Background Acute full revascularization of nonculprit stenoses in STEMI is debated and currently guided by angiography. Acute functional assessment of nonculprit stenoses may be considered. Methods Immediately after successful primary culprit intervention for STEMI, nonculprit coronary stenoses were evaluated with iFR and left untreated. Follow-up evaluation with iFR was performed at a later stage. iFR <0.90 was considered hemodynamically significant. Results One hundred twenty patients with 157 nonculprit lesions were included. Median acute iFR was 0.89 (interquartile range [IQR]: 0.82 to 0.94; n = 156), and median follow-up iFR was 0.91 (interquartile range: 0.86 to 0.96; n = 147). Classification agreement was 78% between acute and follow-up iFR. The negative predictive value of acute iFR was 89%. Median time from acute to follow-up evaluation was 16 days (IQR: 5 to 32 days). With follow-up within 5 days after STEMI, no difference was observed between acute and follow-up iFR, and classification agreement was 89%. With follow-up ≥16 days after STEMI, acute iFR was lower than follow-up iFR, and classification agreement was 70%. Conclusions Acute iFR evaluation appeared valid for ruling out significant nonculprit stenoses in patients with STEMI undergoing primary percutaneous coronary intervention. The time interval from acute to follow-up iFR influenced classification agreement, suggesting that inherent physiological disarrangements during STEMI may contribute to classification disagreement.