The monopoly made it possible for Google to control parts of the online advertising industry, which harmed advertisers and consumers, according to Judge Leonie Brinkema.
"Google has intentionally engaged in a range of anti-competitive actions to acquire and maintain a monopoly on the market," says Brinkema in the ruling.
The court states that it will initiate a process to determine measures for the company, while Google announces that it intends to appeal.
The company claims to compete fairly and believes that the case is based on an outdated view of the digital advertising market.
This is the second time in eight months that Google has been accused of monopoly by an American judge, writes The Wall Street Journal. In August last year, it was deemed clear that Google had exploited its dominance to stop competition.
Google's share initially fell by around 1.5 percent after the announcement.
