Introduction The State Plan Index is an evaluation instrument that uses a Likert scale to assess 60 indicators of the quality of state public health plans. The State Plan Index was needed to enable evaluation of plans that were developed using a variety of public health planning models. Methods Federal, state, and academic partners participated in developing and testing the instrument. The authors conducted a literature review, interviews with experts, and several rounds of formative evaluation to assess item inclusion, coverage, weighting, organization of items, and content validity. In two rounds of field testing, public health practitioners at the federal and state levels rated 10 state public health plans for obesity prevention. Results Field-test raters took an average of two hours to rate a plan and indicated that the State Plan Index was easy to use, comprehensive, and fair. Mean Cronbach a for components of the State Plan Index was 0.88 (median 0.93). Component scores among the 10 plans rated ranged from 0.2 to 4.8, indicating that raters made distinctions in quality among the components and the plans they rated. Correlations between component scores and overall scores were statistically significant (P < .001), except for one component. Conclusion Public health professionals at the federal and state levels found the State Plan Index to be a useful tool for evaluating public health plans that were developed by states using various planning approaches. After the field tests, state staff reported adapting the State Plan Index for use as a planning tool, an evaluation tool for local plans, and a self-assessment tool for drafts of state plans. In addition, the State Plan Index can be revised easily for use in other chronic disease areas.