BACKGROUND Blood glucose regulation in women with diabetes may change during and after menopause, which could be attributed, in part, to decreased estrogen levels. PURPOSE To determine the effect of postmenopausal hormone therapy (HT) on HbA1c, fasting glucose, postprandial glucose, and use of glucose-lowering drugs in women with type 1 and women with type 2 diabetes. DATA SOURCES We conducted a systematic search of MEDLINE, Embase, Scopus, the Cochrane Library, and the ClinicalTrials.gov registry to identify randomized controlled trials (RCTs). STUDY SELECTION We selected RCTs on the effect of HT containing estrogen therapy in postmenopausal women (≥12 months since final menstrual period) with type 1 or type 2 diabetes. DATA EXTRACTION Data were extracted for the following outcomes: HbA1c, fasting glucose, postprandial glucose, and use of glucose-lowering medication. DATA SYNTHESIS Nineteen RCTs were included (12 parallel-group trials and 7 crossover trials), with a total of 1,412 participants, of whom 4.0% had type 1 diabetes. HT reduced HbA1c (mean difference −0.56% [95% CI −0.80, −0.31], −6.08 mmol/mol [95% CI −8.80, −3.36]) and fasting glucose (mean difference −1.15 mmol/L [95% CI −1.78, −0.51]). LIMITATIONS Of included studies, 50% were at high risk of bias. CONCLUSIONS When postmenopausal HT is considered for menopausal symptoms in women with type 2 diabetes, HT is expected to have a neutral-to-beneficial impact on glucose regulation. Evidence for the effect of postmenopausal HT in women with type 1 diabetes was limited.