: Compared to clinic blood pressure (BP), sleep-time BP and non-dipping BP pattern are better predictors of target organ damage and cardiovascular sequalae.: In a retrospective study, we determined whether diabetes mellitus (DM) status is associated with high sleep-time BP and non-dipping pattern.: We analyzed 1092 patients who underwent ambulatory BP monitoring between 2015 and 2017 in a tertiary cardiology institution. During a 24-hour period, BP was automatically measured every 15 minutes between 7:00 AM and 11:59 PM and every 30 minutes thereafter.: Compared with the non-DM group (n = 910), the DM group (n = 182) had a higher 24-hour systolic BP (137 ± 17 vs. non-DM, 132 ± 14 mmHg, p < 0.001) and sleep-time systolic BP (132 ± 20 vs. 123 ± 16 mmHg, p < 0.001), and was more likely to exhibit non-dipping (63% vs 42%, p˂0.001). The DM group was also less likely to meet the guideline-recommended target of 120/70 mmHg for the sleep-time BP measured via ambulatory monitoring (22% vs. 34%, p = 0.002). After adjusting for the effects of age, sex, body mass index, smoking, urea, eGFR, previous myocardial infarction, previous percutaneous coronary intervention, previous coronary artery bypass surgery, and previous stroke, DM remained a significant independent predictor of a higher 24-hour systolic BP (coefficient: 2.8, 95% confidence interval: 0.1-5.5, p = 0.042) and higher sleep-time systolic BP (coefficient: 4.2, 95% confidence interval: 1.1-7.3, p = 0.008). There was a trend toward more sleep-time non-dipping BP pattern (odds ratio: 1.4, 95% confidence interval: 1.0-2.0, p = 0.087) in the DM group.: DM is independently associated with suboptimal 24-hour BP control. This association is mainly attributed to a high sleep-time systolic BP.