Self-reported condom use may be prone to social desirability bias. Our aim was to assess the validity of self-reported condom use in a population of female sex workers using prostate specific antigen (PSA) as a gold standard biomarker of recent unprotected vaginal intercourse. We collected data on 223 sex-workers in Conakry, Guinea in order to assess the sensitivity and specificity of self-reported condom use as well as to examine the predictors of discordance between self-report and PSA presence. PSA was detected in 38.4% of samples. Sensitivity of self-reported condom use was 14.6% and its specificity was 94.7%. Self-perceived high risk of HIV infection was the only significant independent predictor of misreported condom use. PSA could be useful to validate self-reported condom use in surveys and to allow a better understanding of factors associated with social desirability in sexual behaviour reporting.