Researchers have acknowledged that parents play a pivotal role in their children’s autism spectrum disorder (ASD) treatment (Burrell & Borrego, 2012). Unfortunately, many measures of parent involvement for this population use fewer than 10 items and focus only on parents’ reinforcement of therapy skills at home, planning with clinicians, and/or knowledge about treatments (Burrell & Borrego, 2012; Solish & Perry, 2008). A measure of parent involvement that includes all the theorized ways parents support their children’s ASD treatment is necessary for researchers to investigate how parent involvement is related to ASD treatment outcomes. In this study, the validity and reliability of the scores of a new measure for parents’ involvement in their children’s ASD treatment, the Parent Involvement Survey for Autism Treatment-Version 2 (PISAT-2), were investigated using item response theory. Results indicated that PISAT-2 scores had good internal consistency and differentiated well among four of the five proposed levels of parent involvement. Scores from the PISAT-2 were found to be positively correlated with parent self-efficacy (r = 0.38), but not correlated with parent stress. Scores from the PISAT-2 were negatively related to parent education level and annual income, but not related to gender, ethnicity, number of caregivers in the home, reported SES, or work hours per week. The difficulty levels of the PISAT-2 items were similar for participants irrespective of gender, ethnicity, SES, annual income, or work hours per week. However, one item was found to be more difficult for participants with some high school as their highest level of education completed. Participant feedback indicated that the PISAT-2 covered the majority of the ways in which parents are involved in their children’s treatment with some suggestions for improvement.