The high proportion of asymptomatic and undetected SARS-CoV-2 infections presents a challenge to tracking the progress of the pandemic and implementing control measures in Kenya. We determined the prevalence of IgG to SARS-CoV-2 in residual blood samples from mothers attending antenatal care services at 2 referral hospitals in Kenya. A total of 196 samples were analysed from Kenyatta National Hospital in Nairobi in August 2020, seroprevalence, adjusted for assay sensitivity and specificity, was 49.8% (95% CI 42.0-57.8). In Kilifi County Hospital in coastal Kenya, 419 samples were analysed between September and November 2020, seroprevalence, adjusted for assay sensitivity and specificity, increased from 1.3% (95% CI 0.03-4.8) in September to 10.9% (95% CI 6.1, 16.8) in November 2020. There has been substantial, unobserved transmission of SARS-CoV-2 in parts of Nairobi and Kilifi Counties.