Reason Racer is an online, rate-based, multiplayer game that applies specific game features in order to engage middle school students in introductory knowledge of and thinking related to scientific argumentation. Game features include rapid and competitive play, timed performance, immediate feedback, and high rates of response across many game-play sessions and science scenarios. The areas of argumentation addressed in the game include understanding a claim, judging evidence about a claim based on type (fact, opinion) and quality, determining the reasoning applied to the claim (authority, theory, or logic), considering counterarguments and rebuttals, and making judgments. These skills have been identified as important by previous research. Students who played the game at least 10 times improved in every aspect of argumentation skill and judgment. Students who played the game also reported an increase in confidence and motivation to engage in science compared to students who did not play the game. This study has implications for the use of rate-based, multiplayer, competitive games as a component of instruction for difficult-to-teach skills. We also assume that the engaging and fun aspects of Reason Racer contributed to students reporting an increased interest in science.