Background/Aims Postnatal care is offered to mothers and their babies from birth and across the postnatal period. Visits are spread over the postnatal period, and a minimum of four visits is recommended. In many studies, postnatal visits in Africa have been reported to be low compared to antenatal visits. As a result of low postnatal visits, mothers are not able to utilise postnatal care services, resulting in delayed detection of and interventions for maternal and neonatal health problems, leading to high rates of maternal and neonatal morbidity and mortality. In Kenya, only 53% of mothers attend postnatal clinics; in Kakamega county, only 34% of mothers attend. This study aimed to establish factors influencing postnatal knowledge among mothers in selected hospitals in Kakamega, Kenya. Methods The study was a descriptive cross-sectional study involving 320 postnatal mothers recruited from four sub-counties. Systematic sampling was used to select eligible study participants. Data were collected using questionnaires that assessed the participants’ knowledge of postnatal care in terms of what postnatal care is, recommended postnatal care, when to attend a clinic and the services offered at postnatal care clinics. The data were entered into a database and analysed using the Chi-squared test to assess how sociodemographic and socioeconomic characteristics were associated with knowledge of postnatal care. Results The majority of participants (73.1%) had poor or no knowledge of postnatal care and 89.7% had poor or no knowledge on when postnatal visits should be carried out. Most postnatal mothers (71.9%) received postnatal health information from health workers. Occupation (P<0.000), income (P<0.000), transport (P<0.000) and time taken to travel to hospital (P=0.034) were significantly associated with postnatal knowledge. Conclusions Knowledge on postnatal care is poor among postnatal mothers in Kakamega. The majority of participants obtained postnatal care information from health workers, and so it is recommended that Kakamega establishes other strategies for giving information on postnatal care, such as pamphlets to mothers.