Diabetes mellitus (DM) is a prominent global health challenge, characterized by a rising prevalence and substantial morbidity and mortality, especially evident in developing nations. Although DM can be managed with self-care practices despite its complexity and chronic nature, the persistence of poor self-care exacerbates the disease burden. There is a dearth of evidence on the level of poor self-care practices and contributing factors among patients with DM in the study area. Thus, this study assessed the proportion of poor self-care practices and contributing factors among adults with type 2 DM in Adama, Ethiopia. An institution-based cross-sectional study was conducted among 404 patients. Self-care practice was assessed by the summary of diabetes self-care activities questionnaires. Binary logistic regression was used to identify factors associated with poor self-care practices. An adjusted odds ratio with a 95% confidence interval was used to assess the strength of associations. The statistical significance was declared for a p-value < 0.05. The proportion of poor self-care practices was 54% [95% CI 49.1, 58.6]. Being divorced (AOR = 3.5; 95% CI 1.0, 12.2), having a lower level of knowledge (AOR = 1.70; 95% CI 1.0, 2.8), being on insulin (AOR = 6.3; 95% CI 1.9, 20.6), taking oral medication (AOR = 8.6; 95% CI 3.0, 24.5), being unaware of fasting blood sugar (AOR = 2.9; 95% CI 1.6, 5.2), not a member of a diabetic association (AOR = 3.6; 95% CI 1.7, 7.5), a lack of social support (AOR = 2.9; 95% CI 1.7, 4.9), and having a poor perceived benefit of self-care practices (AOR = 1.84; 95% CI 1.0, 3.2) were associated with poor self-care practices. Overall, this finding demonstrated that a significant percentage of participants (54%) had poor self-care practices. Being divorced, having a low level of knowledge about diabetes and fasting blood sugar, lacking social support, relying on oral medication, perceiving limited benefits from self-care practices, and not being a member of diabetic associations were identified as independent factors of poor self-care.