Erectile dysfunction is a complex and common complication of diabetes mellitus, which lacks an effective treatment. The repairing role of vascular endothelium is the current research hotspot of diabetic mellitus erectile dysfunction (DMED), and the activation of PI3K/AKT/eNOS pathway positively affects the repair of vascular endothelium. The herbal extract isorhamnetin has significant vasoprotective effects and has great potential in treating DMED. This study aimed to clarify whether isorhamnetin has an ameliorative effect on DMED and to investigate the modulation of the PI3K/AKT/eNOS signaling pathway by isorhamnetin to discover its potential mechanism of action. In vivo experiments were performed using a streptozotocin-induced diabetic rat model, and efficacy was assessed after 4 weeks of isorhamnetin gavage administration at 10 mg/kg or 20 mg/kg. Erectile function in rats was assessed by maximum intracavernous pressure/mean arterial pressure (ICPmax/MAP), and changes in corpus cavernosum (CC) fibrosis, inflammation levels, oxidative stress levels, and apoptosis were assessed by molecular biology techniques. In vitro experiments using high glucose-induced corpus cavernosum endothelial cells were performed to further validate the anti-apoptotic effect of isorhamnetin and its regulation of the PI3K/AKT/eNOS pathway. The findings demonstrated that isorhamnetin enhanced erectile function, decreased collagen content, and increased smooth muscle content in the CC of diabetic rats. In addition, isorhamnetin decreased the serum levels of pro-inflammatory factors IL-6, TNF-α, and IL-1β, increased the levels of anti-inflammatory factors IL-10 and IL-4, increased the activities of SOD, GPx, and CAT as well as the levels of NO, and decreased the levels of MDA in corpus cavernosum tissues. Isorhamnetin also increased the content of CD31 in CC tissues of diabetic rats, activated the PI3K/AKT/eNOS signaling pathway, and inhibited apoptosis. In conclusion, isorhamnetin exerts a protective effect on erectile function in diabetic rats by reducing the inflammatory response, attenuating the level of oxidative stress and CC fibrosis, improving the endothelial function and inhibiting apoptosis. The mechanism underlying these effects may be linked to the activation of the PI3K/AKT/eNOS pathway.