Objective: Optimizing maternal diet during pregnancy improves maternal and infant health. This study assessed the effect of an antenatal lifestyle intervention for women with overweight or obesity on dietary patterns during pregnancy and post partum. Methods: This study is a secondary analysis of a randomized controlled trial in which pregnant women (BMI ≥ 25 kg/m2) received lifestyle advice (n = 943) or standard care (n = 924). Dietary pattern analysis was undertaken using factor analysis with comparisons between trial entry and 28 weeks, 36 weeks, and 4 months post partum. Results: The dietary pattern analysis revealed two distinct patterns: “prudent” and “Western.” There was a significant difference between groups in the change over time for both patterns (P < 0.001). For the prudent score, the lifestyle group had higher scores at all times after trial entry compared with standard care, while for the Western score, the lifestyle group had a lower score at 28 weeks. The intervention effect differed based on trial entry BMI (P = 0.043) and smoking (P = 0.019), with higher prudent scores for women with obesity compared with overweight and smokers compared with nonsmokers. Conclusions: The provision of an antenatal lifestyle intervention for women with overweight and obesity was associated with an improvement in dietary patterns that persisted post partum. This has important implications for the future consideration of optimal dietary intervention components to include in antenatal lifestyle interventions.