Background: Disclosure of HIV serostatus to sexual partner does not only facilitate partner's support and testing, but it also correlates with better treatment outcomes. We drew from a cross-sectional survey conducted among postpartum women with HIV to examine the rate of serostatus disclosure to sexual partners, the influence of relationship status, and knowing one's partner's status on disclosure and reasons for non-disclosure. Methods: We conducted telephonic interviews with a final sample of 485 postpartum women with HIV drawn from the East London Prospective Cohort study database between January and May 2018. Disclosure of HIV status to partner was based on self-reporting. We fitted adjusted and unadjusted logistic regression models and also conducted descriptive statistical analyses. Results: Overall, 82.5% of the respondents had disclosed their status to their partners. After adjusting for important covariates (age, education level, employment status, number of years since HIV diagnosis, alcohol use and smoking behaviour), being married [adjusted odds ratio (AOR): 2.66; 95% confidence interval (CI):1.17-6.05] and knowing one's partner's status [AOR: 46.91; 95% CI:19.50-112.81] were significantly associated with a higher odds of having disclosed HIV status to sexual partners. Fear of rejection, stigma or being judged, new or casual relationships, and having a violent partner were the main reasons for not disclosing HIV status to sexual partners. Conclusion: We found a relatively high rate of HIV status disclosure in the cohort. Being married and having open communication with a partner about HIV status are important factors facilitating disclosure to sexual partners.