Homelessness and problem gambling are two public health concerns in the UK that are rarely considered concurrently, and little is known about the extent of gambling involvement and problematic gambling in the homeless. We recruited 456 individuals attending homelessness services in London, U.K. All participants completed a screen for gambling involvement, and where gambling involvement was endorsed, the Problem Gambling Severity Index (PGSI) was administered. The PGSI risk categories were compared against data from the 2010 British Gambling Prevalence Survey (BGPS). PGSI problem gambling was indicated in 11.6% of the homeless population, compared to 0.7% in the BGPS. Of participants endorsing any PGSI symptoms, a higher proportion of homeless participants were problem gamblers relative to the low and moderate risk groups, compared to the BGPS data. These results confirm that the homeless constitute a vulnerable population for problem gambling, and that diagnostic tools for gambling involvement should be integrated into homelessness services in the U.K.