Men who have sex with men (MSM) are at growing risk of HIV infection in many parts of the world; however, the epidemic has not been well explored among this population in most Arab countries. To estimate the prevalence of HIV and related risk behaviors among MSM in Yemen, we recruited 261 adult MSM from the port cities of Aden and Al-Hudaydah through venue- and facility-based sampling. Behavioral data were collected with a face-to-face questionnaire, and HIV status was determined by serological testing. HIV prevalence was 5.9 % (95 % CI 4.8-7.3). One-fourth (25.8 %, 95 % CI 20.7-31.5) had tested for HIV in the last year and received results; 27.8 % (95 % CI 22.5-33.7) had comprehensive knowledge about HIV; 20.0 % (95 % CI 15.8-25.0) reported condom use at last anal sex; and 31.4 % (95 % CI 25.9-37.3) reported that they or their sexual partner had a sexually transmitted disease symptom. Injecting drugs in the last year was reported by 0.8 % (95 % CI 0.1-9.2). Multiple risk behaviors, low HIV knowledge, few preventive behaviors, and HIV prevalence greater than 5 % denote a concentrated and potentially expanding HIV epidemic among MSM in Yemen. No time should be lost in intervening to prevent further expansion of the epidemic to levels already seen among MSM outside the Middle East.