Background: Children are inhabitants of a media-rich environment rife in extensive, sophisticated, and persistent techniques that are used to market unhealthy food. Exposure is known to influence children's attitudes, choices, and consumption, yet further research is required to explore the influence of contemporary techniques within online games. Objective: To explore the influence of modern advertising on children's attitudes, choices, and consumption, techniques (ie, banner advertising, advergame, and rewarded video advertising) were used to promote an unfamiliar confectionery brand within an online game. Design: A between-subjects randomized experimental study. Participants/setting: Children (aged 7 to 12 years [n=156]) were recruited in New South Wales, Australia, between September and November 2017. Intervention: Children were required to play a 4-minute online game, complete some questionnaires, and choose one snack to consume afterward. Children were randomly assigned to one of four conditions: a control group with no advertising, and three experimental conditions that promoted an unfamiliar confectionery brand via a banner advertisement, advergame, or rewarded video advertisement. Main outcome measures: Questionnaires included the assessment of attitudes to the test brand before and after the game, enjoyment of the game, and children's awareness of advertising. Food choice was recorded and food consumption was measured by weighing the snack in grams, which was translated into kilocalories. Statistical analyses performed: Statistical tests included analyses of variance, Kruskal-Wallis test, and χ2 test. Results: Attitudes toward the perception of fun (P=0.06) and taste (P=0.21) of the test brand were not influenced by condition. Children who were exposed to the rewarded video advertising chose the test brand significantly more than children in the other three conditions (P<0.002). Condition did not influence overall energy intake measured in grams (P=0.78) or kilocalories (P=0.46). Conclusions: Children's choice of the test brand was significantly influenced by the rewarded video advertising condition (compared with control, banner advertising, and advergame conditions). This technique is prevalent across online and application games that children play yet the effects of using rewarded video advertising to promote food brands have not been explored from a public health perspective. This study contributes to the understanding of modern strategies used to market unhealthy foods to children.