Reciprocal ST-segment downsloping on electrocardiogram is a frequent finding during ST-elevated myocardial infarction (STEMI), but its etiology is still disputed. We sought to evaluate the relation between reciprocal ST-segment downsloping during STEMI and major cardiac perfusion and functional parameters. One hundred eighty-five patients with STEMI underwent emergency coronary angiography. The presence of reciprocal ST-segment downsloping was assessed. At coronary angiography, the corrected TIMI frame count (cTFC) was computed both on culprit and remote vessels and the occurrence of “no/slow reflow” phenomenon after percutaneous coronary intervention (PCI) identified. The left ventricular wall motion score index ratio (discharge/admission values) at echocardiography and the slope of high-sensitivity troponin elimination were computed as measures of effective myocardial reperfusion. Reciprocal ST-segment downsloping was revealed in 91 patients (49%). They presented higher cTFC values on remote vessels than patients without reciprocal electrocardiographic abnormalities (44 ± 18 vs 37 ± 15 cineframes × second⁻¹, p = 0.004). The presence of remote ST-segment downsloping was also associated with a higher prevalence of “no/slow reflow” phenomenon (59% vs 40%, p = 0.013) as well as more abnormal values of wall motion score index ratio (p = 0.042) and high-sensitivity troponin slope (p = 0.012). At multivariate analyses, a higher cTFC on remote vessels predicted the occurrence of reciprocal ST-segment changes (p = 0.018) and the development of “no/slow reflow” phenomenon after PCI (p = 0.005). In conclusion, the presence of reciprocal ST-segment downsloping during STEMI clusters with significant perfusion and cardiac functional abnormalities, predicting the development of “no reflow” phenomenon after PCI.