We aimed to study whether lifestyle intervention could reduce systemic oxidative stress (OS) and the association between OS and cardiometabolic outcomes in women with obesity and infertility. From 2009 to 2012, infertile women with a BMI ≥ 29 kg/m2 were randomly assigned to a six-month lifestyle intervention followed by infertility treatment (N = 289) or to prompt infertility treatment (N = 285). Fasting serum free thiols (FT) concentrations were determined by colorimetry at baseline, at three and six months after randomization. Generalized estimating equations and restricted cubic spline regressions were used to estimate mean differences in serum FT levels between groups and to explore associations between serum FT levels and cardiometabolic outcomes. Baseline serum FT levels did not differ between the two groups (N = 203 in the intervention group vs N = 226 in the control group, 222.1 ± 48.0 µM vs 229.9 ± 47.8 µM, p = 0.09). Body weight decreased by 3.70 kg in the intervention group compared with the control group at six months (95% confidence interval [CI]: −7.61 to 0.21, p = 0.06). No differences in serum FT levels were observed between groups at either three months (N = 142 vs N = 150, mean differences: −1.03 µM, 95% CI: −8.37 to 6.32, p = 0.78) or six months (N = 104 vs N = 96, mean differences: 2.19 µM, 95% CI: −5.90 to 10.28, p = 0.60). In a pooled analysis of all available measurements, triglycerides (crude B: 5.29, 95% CI: 1.08 to 9.50, p = 0.01), insulin (crude B: 0.62, 95% CI: 0.26 to 0.98, p = 0.001), and homeostasis model assessment of insulin resistance (crude B: 2.50, 95% CI: 1.16 to 3.38, p < 0.001) were positively associated with serum FT levels. High-sensitivity C-reactive protein (hs-CRP) was negatively associated with serum FT levels (crude B: −0.60, 95% CI: −1.11 to −0.10, p = 0.02). The change in hs-CRP during the lifestyle intervention was strongly and inversely associated with serum FT levels (crude B: −0.41, 95% CI: −0.70 to −0.13, p = 0.005). No significant deviations from linear associations were observed between serum FT and hs-CRP. We do not observe an improvement in systemic OS in women with obesity and infertility with modest weight loss. There were potential associations between OS and biomarkers of cardiometabolic health. Trial registration: This trial was registered on 16 November 2008 at the Dutch trial register (NTR1530).