PURPOSE: Women with gestational diabetes mellitus (GDM) or obesity are vulnerable to impaired gestational cardiovascular health (CVH) and cardiovascular disease (CVD) in the future. It is unclear if prenatal vitamin D supplementation improves gestational CVH, especially in women at high risk for developing CVD. Our goal was to find out if vitamin D supplementation could protect against gestational CVH, including the women with GDM or obesity. DESIGN: We randomly assigned women with a serum 25(OH)D concentration < 75 nmol/L to receive 1600 IU/d (intervention group) or 400 IU/d (control group) of vitamin D3 for two months at 24-28 weeks' gestation. The primary outcome was gestational CVH marks (lipids, inflammatory cytokines, endothelial function). RESULTS: There were 1537 participants divided into the intervention (N = 766) and control groups (N = 771). No baseline differences existed among study groups in CVH markers. At the two-month visit, the intervention group's HDL-C levels (2.01 ± 0.39 VS 1.96 ± 0.39 mmol/L) were significantly higher than those of the control group, while the hs-CRP levels were significantly lower (3.28 ± 2.02 VS 3.64 ± 2.42 mg/L). Subgroup analysis found that HDL-C, TC, hs-CRP, E-Selectin, and SBP were improved in the intervention group among women with GDM or overweight/obesity, and the improvement was not found in women without GDM or overweight/obesity. Vitamin D supplementation significantly decreased the mean triglyceride-glucose index at the two-month visit in women with GDM. CONCLUSIONS: Vitamin D supplementation at mid-gestation might optimize the gestational CVH status for pregnant women, particularly the women with GDM or obesity, which is advantageous for later-life primary prevention of CVD. CLINICAL TRIAL REGISTRATION: The Chinese Clinical Trial Registry (ChiCTR2100051914, 10/9/2021, Prospective registered, https://www.chictr.org.cn/showproj.aspx?proj=134700 ).