Background/Objectives: Studies have shown that chronobiological factors may adversely affect glycemic control in patients with type 2 diabetes mellitus. We assessed the association of chronobiological factors with glycemic control and neonatal birth weight in women with GDM. Methods: A prospective cohort study included 208 women aged 18–45 years with a singleton pregnancy who were randomly selected from among women undergoing follow-up for GDM at the Maternal-Fetal Medicine Unit of a tertiary medical center. Nutrition, sleep, and lifestyle patterns were assessed from onset of GDM until birth along with glycemic control and birth outcomes. Results: Multivariate analyses on a cohort of 208 women revealed that suboptimal glycemic control was associated with a late breakfast (RR = 2.26; 95% CI 1.09–4.67), increased carbohydrate intake in the evening (RR = 1.19; 95% CI 1.003–1.42), and poor sleep quality (RR = 2.14; 95% CI 1.04–4.41). The adjusted relative risk for neonatal birth weight above the 85th percentile was associated with increased carbohydrate intake in the morning (RR = 1.70; 95% CI 1.30–2.23) and increased carbohydrate intake in the evening (RR = 1.39; 95% CI 1.16–1.67). Conclusions: Chronobiological factors are associated with suboptimal glycemic control and birth weight above the 85th percentile in women with GDM. The study was registered under ClinicalTrials.gov.org, identifier: NCT02916667.