The number of new HIV infections continues to be on the rise in many high-income countries, most notably among men who have sex with men (MSM). Despite recent attention to the use of antiretroviral medications as pre-exposure prophylaxis (PrEP) among MSM, considerably less research has been devoted to examining the awareness and use of post-exposure prophylaxis (PEP). Based on a convenience sample of 179 self-reported HIV-uninfected MSM using a geosocial-networking smartphone application, this study is among the first to examine the awareness and use of PEP and their demographic and behavioral correlates among MSM in London. Most respondents (88.3%) had heard of PEP, where 27.4% reported having used it. In multivariable models, the disclosure of one's sexual orientation to their general practitioner (Prevalence ratio [PR]: 3.49; 95% confidence interval (CI): 1.14, 10.70; p = .029) and reporting one's HIV status as negative (rather than unknown) (PR: 11.49; 95% CI: 1.68, 76.92; p = .013) were associated with having heard of PEP; while the recent use of club drugs (PR: 3.02; 95% CI: 1.42, 6.43; p = .004) was associated with having ever used PEP. High awareness and use in this sample suggest that PEP is a valuable risk-reduction strategy that should be capitalized on, be it in addition to or in the absence of PrEP.