This study will enroll healthy, HIV-uninfected people. Participants will be randomly assigned to receive either the rAd26 vaccine or a placebo vaccine. All vaccines will be injected into the upper arm. At the vaccination study visit, participants will undergo a medical and medication history review, physical examination, and a rectal sampling procedure. They will then receive their assigned vaccine. After receiving the vaccine, participants will remain in the clinic for at least 30 minutes for observation and monitoring of side effects. Participants will receive counseling on HIV risk reduction and pregnancy prevention. For 7 days after the vaccination, participants will monitor their temperature and side effects. Additional study visits will occur at Days 1, 3, 7, 14, 28, 61, 168, and 365. At these visits, participants will undergo a medical history review, physical examination, counseling, and blood collection. At the Day 14 and 168 visits, a rectal sampling procedure will be performed. Participants will attend a study visit or be contacted by study researchers by telephone or e-mail 18 months after receiving the vaccine for follow-up health and safety monitoring. Participants may elect to attend optional follow-up visits for blood collection at Years 2, 3, 4, and 5.