Abstract Background Annually, 125.2 million pregnant women worldwide risk contracting malaria, including 30.3 million and 1.5 million in Sub-Saharan Africa and Kenya respectively. At least three doses of sulphadoxine pyrimethamine for intermittent preventive treatment of malaria in pregnancy (IPTp-SP) is recommended for optimal benefit. Kenya recorded low IPTp-SP optimal uptake in 2015. This study investigated the prevalence of and factors influencing IPTp-SP optimal uptake in Sabatia Sub County, Western Kenya. Methods A cross-sectional study was conducted in Sabatia Sub County from April to October 2020. Using a validated semi-structured questionnaire, data were obtained from 372 randomly sampled post-delivery women aged 15–49 years with live birth within one year preceding the study. Women on cotrimoxazole prophylaxis during pregnancy were excluded. Pearson Chi-square and Fisher’s Exact test were measures of association used. Binary logistic regression analysed predictors of optimal IPTp-SP uptake. Results Optimal IPTp-SP uptake was 79.6%, 95% CI 75.5%–83.7%. Predictors of IPTp-SP optimization were gestational age at first antenatal care (ANC) visit ( P = 0.04), frequency of ANC visits ( P < 0.001), maternal knowledge of IPTp-SP benefits ( P < 0.001), maternal knowledge of optimal sulphadoxine pyrimethamine (SP) dose ( P = 0.03) and SP administration at ANC clinic ( P = 0.03). Late ANC initiators were less likely to receive optimal IPTp-SP (a OR = 0.4, 95% CI 0.2–0.9). Odds of optimizing IPTp-SP increased among women with ≥ 4 ANC visits (a OR = 16.7, 95% CI 7.9–35.3), good knowledge of IPTp-SP benefits (a OR = 2.4, 95% CI 1.3–4.5) and good knowledge of optimal SP dose (a OR = 1.9, 95% CI 1.1–3.4). Women who never missed being administered SP were highly likely to receive optimal IPTp-SP (a OR = 2.9, 95% CI 1.1–7.2) Conclusions This study has found high IPTp-SP optimal uptake in the study area. Efforts should be directed towards early and more frequent ANC visits. Intensive and targeted health education ...