Background: The effective approach to preventing preeclampsia (PE) is administering acetylsalicylic acid (ASA) to high-risk patients. However, there are not enough data analyzing the effectiveness of ASA intake by pregnant women with diabetes mellitus (DM). This study aims to evaluate the effect of ASA on perinatal outcomes in pregnant women with different types of pregestational DM. Methods: This retrospective study included 735 pregnant women with DM (types 1 and 2). At 12–14 weeks of gestation, some patients were prescribed daily ASA at a 100–150 mg dose continuously for up to 36 weeks. The effect of ASA on the development of PE and other outcomes of pregnancy was assessed. The times of delivery and the onset of PE were evaluated as well. Results: When taking ASA, PE developed significantly less frequently in pregnant women with DM. This was significantly more evident in patients with type 2 DM (OR 0.65; 95% CI: 0.52–0.79). In patients with type 1 DM, the mean period of development of PE was 1.5 weeks later relative to those pregnant women who did not take the drug and was 35.5 weeks of gestation. The OR for the development of preterm birth was reduced by 3 times (OR 0.33; 95% CI: 0.15–0.62). In women with DM who took ASA during pregnancy, babies were born with greater body weight, and the frequency of small for gestational age births decreased. Conclusions: ASA administration is associated with a reduction of the incidence of PE, a delay in its manifestations, and a mitigating the risk of other adverse perinatal outcomes typical for pregnant women with DM.