Theories of belief system structure and dynamics assume that belief systems are a person-level construct. However, measures of belief system structure do not measure the structure of person-level belief systems and instead measure aggregated belief system structure (e.g., the belief system in a particular country). In this paper, I show that a measure of conceptual similarity between attitudes and identities of a belief system works as a valid, reliable, flexible, and efficient measure of person-level belief system structure in the United States. In Studies 1 (N = 387), 2 (N = 389), and 3 (N = 598), I show conceptual similarity judgments are reliable and are related to measures of political engagement, political knowledge, attitude consistency, and preference congruence as predicted by computational models of belief system dynamics. In Studies 4 (N = 981) and 5 (N = 983), I show that conceptual similarity judgments are affected by partisan frames and that changes in conceptual similarity judgments are associated with attitude change as predicted by computational models of belief system dynamics. Conceptual similarity judgments can be used with a variety of attitudes and identities in easy to administer studies. It provides a tool to fill an empirical gap identified by theories of belief system dynamics. (PsycInfo Database Record (c) 2022 APA, all rights reserved).