The purpose of this study was to develop the Human-Animal Interaction Scale (HAIS) and evaluate its reliability and validity. The HAIS is a 24-item self-report instrument designed to describe and quantify behaviors performed by humans and nonhuman animals during an episode of interaction (e.g., engaging with a pet, participating in an animal-assisted intervention). Participants were 295 adult volunteers who completed the HAIS in one of several different contexts, including both laboratory and applied settings. The scale was tested across several different species, including companion animals (i.e., dogs and cats), small caged animals (i.e., rats, rabbit, hedgehog), and horses. Analyses indicate good reliability, with a Cronbach's alpha of 0.82 overall and alphas of 0.72 and higher across the different species and settings. Test-retest analyses indicate ratings remain consistent up to one week following an interaction. Evidence of construct validity was gathered by comparing HAIS ratings with other well-established measures of related constructs, as well as comparing participant reports with researcher observations. Potential uses in basic and applied research are discussed.