People with intellectual disabilities (IDs) have a higher risk of painful medical conditions. Partly because of the impaired ability to communicate about it, pain is often undertreated. To strengthen pain assessment in this population, we conducted a systematic review to identify behavioral pain indicators in people with IDs by using Embase, PubMed, PsycINFO, CINAHL, and Cochrane. Inclusion criteria were 1) scientific papers; 2) published in the last 20 years, that is, 1992 to 2012; 3) written in English, 4) using human subjects, 5) intellectual disabilities, 6) pain, 7) behavior, and 8) an association between observable behavior and pain experience. From 527 publications, 27 studies were included. Pain was acute in 14 studies, chronic in 2 studies, both acute and chronic in 2 studies, and unspecified in 9 studies. Methodological quality was assessed with the Mixed Methods Appraisal Tool. Of the 14 categories with behavioral pain indicators, motor activity, facial activity, social-emotional indicators, and nonverbal vocal expression were the most frequently reported. Most of the behavioral pain indicators are reported in more than 1 study and form a possible clinical relevant set of indicators for pain in people with IDs. Determination of a behavioral pattern specific for pain, however, remains a challenge for future research.