A federal judge ruled today that Google is a monopolist in some parts of the online advertising market, marking the second case in a year where the company was found to have violated US antitrust law. Last August, a federal judge ruled that Google was maintaining an illegal monopoly in search.

Judge Leonie Brinkema of the US District Court for the Eastern District of Virginia determined that Google illegally monopolized parts of its advertising technology business to dominate the programmatic ad market, a major source of revenue for the company. Google generated nearly $30.4 billion in worldwide revenue last year from placing ads on other apps and websites. Now, a substantial portion of those sales are threatened by penalties that may follow Brinkema’s ruling. A best-case scenario for US consumers is a browsing experience filled with fewer ads and paywalls and more content choices.

“In addition to depriving rivals of the ability to compete, [Google’s] exclusionary conduct substantially harmed Google’s publisher customers, the competitive process, and, ultimately, consumers of information on the open web,” Brinkema wrote.

Google was found to have violated Section 2 of the Sherman Act, the cornerstone antitrust law in the US, “by willfully acquiring and maintaining monopoly power in the open-web display publisher ad server market and the open-web display ad exchange market, and has unlawfully tied its publisher ad server (DFP) and ad exchange (AdX).” In other words, the way that Google tied parts of its ad tech together was deemed unlawful.

Online ads end up in front of consumers after passing through a chain of systems linking publishers to advertisers. Google has long been viewed as a dominant provider of tools at nearly every step in this process, which critics argue enables the company to give preferential trea...