Background: Self medication in pregnancy has continued to be on the front burner of the global public health discourse due to the increased incidence of abortion, foetal malformation and antimicrobial resistance that were found to be associated with it. Inspite of this burden, in Nigeria, most studies on self-medication were conducted on the general population with only a few carried out among pregnant women. Therefore, this study was carried out to determine the prevalence and factors associated with self-medication among pregnant women in Oyo State. Methods: The study was a hospital-based cross-sectional survey. A multi-stage sampling technique was used to select 347 pregnant women who attended antenatal clinics at 56 primary health care facilities in Ogbomoso. A semi-structured, interviewer-administered questionnaire was used to obtain information on the practice of self-medication. Data were analyzed using SPSS version 25; bivariate analysis was done using Chi-square and binary logistic regression was used to identify the predictors of self-medication in pregnancy. The level of statistical significance was set at p < 0.05. Results: The mean age of the respondents was 27.0 ± 6 years. The majority (76.6%) of the respondents had poor knowledge of self-medication. About half of the respondents self-medicated with the analgesics (83.1%), antibiotics (66.7%) and antimalarial drugs (28.9%). Nausea and vomiting (90.4%), diarrhoea (67.8%), back pain (62.1%) and fever (28.8%) were the common medical problems. The main reasons for self-medication were previous experience with the drugs (50.9%), the suggestion by family about the drugs (45.2%) and advice from the community pharmacists (31.6%). The proportion of the respondents who used herbal concoction was 30.0% and the reasons given for the herbal use were low cost (47.6%) and perceived efficacy (42.9%). The proportion of pregnant women who gave a history of abnormality in the previously born baby (ies) was 13.0%. Cleft lip (66.7%) was the most prevalent abnormality. The determinants of self-medication were age (aOR: 0.7, 95% CI: 0.33- 0.97), education (aOR: 0.7, 95% CI: 0.49 - 0.99), confidence in other health workers’ prescription (aOR: 0.4, 95% CI: 0.15 – 0.88), relief from discomfort (aOR: 2.8, 95% CI: 1.46 – 5.42) and suggestion by families (aOR: 3.6, 95% CI: 1.62 – 8.05). Conclusion: The prevalence of self-medication was high and was more pronounced among married, urban dwellers and those with post-basic education including the respondents aged 25 years and above. This may be explained by the higher proportion of the respondents with poor knowledge of self-medication. Health education, especially during antenatal clinics, on the dangers associated with self-medication in pregnancy, is therefore recommended.