The Treatment Adherence Perception Questionnaire (TAPQ) is a new, brief self-report instrument for assessing patient perceptions and attitudes regarding their own adherence to medical treatment plans. It includes 3 distinct scales: Perceived Behavior, Perceived Benefit, and Perceived Burden. In contrast with existing measures, the TAPQ was expected to have a clear factor structure; have good discrimination; and assess distinct types of perception, each of which has different patterns of association with interpersonal, personality, motivational, and emotional variables. Foundational work on the TAPQ (with 891 patients) included 5 quantitative scale development studies and 1 qualitative study. The present report focuses on results from a final validation study using 450 patients recruited via market research panels to complete online questionnaires. This study included a general medical sample and a sample of people with either diabetes or hypertension. A confirmatory factor analysis specifying strict measurement invariance across these groups produced a good fit. Analysis with item-response theory suggested that the scales on the TAPQ provide good discrimination across a wide range of experience levels. The 3 scales on the TAPQ each had distinct patterns of association with criterion variables regarding conscientiousness, health behavior, motivation, affect, type of diagnosis, and interpersonal communication with health-care professionals. These effects could not be explained by another existing measure of adherence or by a measure of response bias. (PsycINFO Database Record (c) 2020 APA, all rights reserved).