Youth advocacy for obesity prevention is a promising but under-evaluated intervention. The aims of this study are to evaluate a youth advocacy program's outcomes related to youth perceptions and behaviors, develop an index of youth advocacy readiness, and assess potential predictors of advocacy readiness. Youth ages 9-22 in an advocacy training program (n = 92 matched pairs) completed surveys before and after training. Youth outcomes and potential predictors of advocacy readiness were assessed with evaluated scales. All 20 groups who completed the evaluation study presented their advocacy projects to a decision maker. Two of six perception subscales increased following participation in the advocacy program: self-efficacy for advocacy behaviors (p < .001) and participation in advocacy (p < .01). Four of five knowledge and skills subscales increased: assertiveness (p < .01), health advocacy history (p < .001), knowledge of resources (p < .01), and social support for health behaviors (p < .001). Youth increased days of meeting physical activity recommendations (p < .05). In a mixed regression model, four subscales were associated with the advocacy readiness index: optimism for change (B = 1.46, 95 % CI = .49-2.44), sports and physical activity enjoyment (B = .55, 95 % CI = .05-1.05), roles and participation (B = 1.81, 95 % CI = .60-3.02), and advocacy activities (B = 1.49, 95 % CI = .64-2.32). The youth advocacy readiness index is a novel way to determine the effects of multiple correlates of advocacy readiness. Childhood obesity-related advocacy training appeared to improve youths' readiness for advocacy and physical activity.