BACKGROUND In a world increasingly dependent on the Internet for information, it is not surprising that people use the internet to find answers to their health-related questions. Research has shown that teen girls are more likely to search for health information online than boys, but that they do not feel confident in using the information they find. OBJECTIVE To address this disparity, teen girls were engaged in the process of developing a teen-friendly, internet-based tool that explains the best way to find, evaluate, and use online health information. METHODS Focus groups were held with girls and their parents to inform the design of the tool. After collaborating with information technology, marketing, and video production teams, a tool was developed consisting of a webpage, videos, and an interactive game. The efficacy and acceptability of the tool were tested among our target demographic through a usability trial. RESULTS Parent and teen focus groups informed the three-step design of the tool. Teen girls reported significantly higher levels of eHealth literacy after using the tool. Dissemination of the tool through a national targeted ad campaign generated web traffic to the tool. CONCLUSIONS An internet-based training tool has been shown to improve teen girls' eHealth literacy. Findings from this developmental study can be used to inform efforts to improve eHealth literacy in adolescent girls. [HLRP: Health Literacy Research and Practice. 2021;5(1):e26-e34.] PLAIN LANGUAGE SUMMARY: This research study used teen and parent input to help design an internet-based training tool to teach teen girls the best way to use the internet to find health information. Teen girls' ability to find, understand, and evaluate online health information significantly improved after using the tool. The tool was advertised throughout the United States on various social media platforms.