Background: In the emergency department (ED), hyperglycemia may be overlooked due to non-diabetes mellitus (DM) primary diagnoses. We compared the risk of all-cause hospital revisits within 30 days after ED discharge in DM patients with normal blood glucose (BG), moderate hyperglycemia, and severe hyperglycemia. Methods: This was a retrospective cohort study of patients 18 years and older discharged from a tertiary care ED between 1 January and 31 March 2018. The severe hyperglycemia group had BG levels of 300 mg/dL (16.7 mmol/L) or greater. The moderate hyperglycemia group had a history of DM, all BG levels less than 300 mg/dL (16.7 mmol/L), and at least one BG level of 180 mg/dL (10 mmol/L) or greater. The normal BG group had a history of DM and BG less than 180 mg/dL (10 mmol/L). Results: Of 302 patients who met criteria, 118 had severe hyperglycemia, 67 had moderate hyperglycemia, and 117 had normal BG. No significant difference between the severe hyperglycemia, moderate hyperglycemia, and normal BG groups was found in 30-day all-cause hospital revisits (19.5% vs. 10.4% vs. 15.4%, respectively, p = 0.25). Patients with a past medical history (PMH) of atherosclerotic cardiovascular disease (ASCVD) or any ED visit in the year preceding the index visit each had an increased risk of a hospital revisit within 30 days (p = 0.025) after covariate adjustment; the adjusted risk of a 30-day hospital revisit among those with a PMH of ASCVD was 2.68 times greater than the risk among those without a history of ASCVD (95% CI: 1.59 to 4.53), and the adjusted RR of a 30-day revisit among those who had an ED visit in the prior year was 1.92 times greater than those without an ED visit in the prior year (95% CI: 1.10 to 3.35). Conclusions: The results suggest no significant association between hyperglycemia in the ED and 30-day hospital revisits. In any patient with DM with a history of ASCVD or any ED visit in the previous year, there may be an increased risk of revisits.