Background: Promoting COVID-19 prevention is key to pandemic control and innovative interventions can help communicate reliable science to the public. Under the Hong Kong Jockey Club SMART Family-Link Project, we developed and evaluated a pilot intervention for promoting COVID-19 prevention through a web-based family game, guided by the Theory of Planned Behavior and a strength-based approach. Methods: The "SMART Epidemic prevention" pilot theme was launched to the public on September 21, 2020 for 4 weeks. The game had two parts: (i) strength recognition and (ii) quiz questions on knowledge and behaviors about COVID-19 prevention. Simple baseline, in-game, and postgame evaluation assessed players' perceived knowledge, behaviors, family well-being, game satisfaction, and perceived benefits. Results: Of 86 registered families, 55 played actively, including 212 players (51% female, 35% aged below 18) who self-identified as children (44%), parents (39%), and grandparents (11%). In weeks 1 and 4, an average of 7 and 18 game rounds were played per family, and 86.6% and 75.9% of rounds had perfect (2) behavior matches. Postgame evaluation with 51 families showed improvements in epidemic prevention knowledge and behaviors, family communication, family happiness (all P < 0.001), and family relationship (P = 0.002) with small effect sizes (0.15-0.29). Overall game satisfaction was rated 4.49 (scale of 1-5). Ninety-four percent of families shared knowledge from the game with others. Conclusions: Our pilot web-based family game first showed preliminary evidence on enhancing COVID-19 prevention knowledge and behaviors, and family well-being, with participants recognizing family strengths, reporting high satisfaction and various perceived benefits, and showing sustained gameplay. Trial Registration: The research protocol was registered at the National Institutes of Health (Identifier No. NCT04550065) on September 16, 2020.