The impact of diabetes mellitus (DM) on outcomes of patients with ST-segment elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PCI) was confirmed by several studies. However, it is unclear whether this effect is still present in large groups of unselected patients undergoing up-to-date treatment. Thus, we sought to assess the impact of DM on periprocedural outcomes of primary PCI in STEMI using data from the Polish National Registry of PCI. Data on 150,782 STEMI patients undergoing primary PCI were collected. Of them, 26,360 (17.5%) patients had DM. Patients with DM were higher-risk individuals who experienced longer reperfusion delays and were less likely to have closed infarct-related artery at baseline (TIMI 0 + 1 flow: 73.2% vs. 72.0%; p < 0.0001) and achieve optimal reperfusion after PCI (TIMI 3 flow: 91.8% vs. 88.5%; p < 0.0001). The periprocedural mortality (1.1% vs. 1.9%; p < 0.0001) was higher in patients with DM and DM was identified as an independent predictor of periprocedural death. In conclusion, despite continuous progress in STEMI treatment, DM remains a strong predictor of periprocedural mortality. However, this detrimental effect of DM may be partially explained by the overall higher risk profile of diabetic patients.