Objective The aim of this study is to evaluate the efficacy of dienogest versus levonorgestrel-releasing intrauterine system (LNG-IUS) for the treatment of adenomyosis. Methods In this retrospective study, 85 patients with adenomyosis treated in The First Affiliated Hospital of Zhengzhou University from May 2019 to May 2021 were recruited and assigned, via the random number table method at a ratio of 1 : 1, to receive either dienogest (observation group, n = 41) or LNG-IUS (control group, n = 44). The patients presented with dysmenorrhea, menorrhagia, and infertility. The treatment outcome was evaluated using visual analogue scale (VAS) scores, menstrual volume, uterine volume, endometrial thickness, and adverse reactions. Results After treatment, the VAS score, menstrual volume, and endometrial thickness were significantly decreased in both groups (P < 0.05). After 3, 6, and 12 months of treatment, patients receiving dienogest showed significantly lower VAS scores compared to those treated with LNG-IUS (P < 0.05). After 6 and 12 months of treatment, patients receiving dienogest were also found to have a significantly better control of menstrual volume compared to those receiving LNG-IUS (P < 0.05). Irregular vaginal bleeding was mainly seen in the first 3 months of treatment with dienogest. The incidence of irregular vaginal bleeding lasting more than 6 months was lower with LNG-IUS treatment than with dienogest (P < 0.05), and it decreased in both groups as the duration of treatment increased. Conclusion Dienogest effectively alleviates dysmenorrhea, relieves pelvic pain, dyspareunia, and reduces menstrual flow in patients with adenomyosis, with few adverse effects and a high safety profile.