Elementary school students often exhibit a variety of conceptions associated with algebraic thinking that their teachers fail to recognize or understand. It is crucial that elementary school teachers possess knowledge of the variety of student conceptions and also have abilities to address varying states of conceptions. Otherwise, students who are not provided with opportunities to think algebraically may continue to struggle with algebra throughout school. The purpose of this study, therefore, was to develop and provide initial validity evidence for an assessment tool of algebraic thinking for the elementary grades. In the development of the tool, validation efforts were pursued, including multiple rounds of pilot-testing and think aloud interviews. Evidence for reliability of scores and validity of score interpretations of the overall score were collected. Results suggest that the assessment is a potentially valuable tool for teachers to elicit and learn about students' algebraic thinking knowledge.