Adenoviruses (AdVs) are DNA viruses that infect many vertebrate hosts, including humans and nonhuman primates. Here we identify a novel AdV species, provisionally named “simian adenovirus C (SAdV-C),” associated with a 1997 outbreak of acute respiratory illness in captive baboons (4 of 9) at a primate research facility in Texas. None of the six AdVs recovered from baboons (BaAdVs) during the outbreak, including the two baboons who died from pneumonia, were typeable. Since clinical samples from the two fatal cases were not available, whole-genome sequencing of nasal isolates from one sick baboon and three asymptomatic baboons during the outbreak was performed. Three AdVs were members of species SAdV-C (BaAdV-2 and BaAdV-4 were genetically identical, and BaAdV-3), while one (BaAdV-1) was a member of the recently described SAdV-B species. BaAdV-3 was the only AdV among the 4 isolated from a sick baboon, and thus was deemed to be the cause of the outbreak. Significant divergence (<58% amino acid identity) was found in one of the fiber proteins of BaAdV-3 relative to BaAdV-2 and -4, suggesting that BaAdV-3 may be a rare SAdV-C recombinant. Neutralizing antibodies to the other 3 AdVs, but not BaAdV-3, were detected in healthy baboons from 1996 to 2003 and staff personnel from 1997. These results implicate a novel adenovirus species (SAdV-C) in an acute respiratory outbreak in a baboon colony and underscore the potential for cross-species transmission of AdVs between humans and nonhuman primates.