A federal judge has ruled that Google (GOOG, GOOGL) holds an illegal monopoly over the online advertising business, its second antitrust loss in more than a year. 

The Justice Department and a collection of 17 states brought the case against Google in Jan. 2023 and went to trial in September, claiming that Google violated antitrust laws in three markets: the publisher ad server market, the ad exchange market, and the advertiser ad network market.

In her ruling on Thursday, US District Court Judge Leonie Brinkema said that the plaintiffs proved Google held monopoly power in the publisher ad servers and ad exchanges for open-web display advertising. But Brinkema found that the DOJ and states failed to show a relevant market for advertising networks.

Shares of Google parent Alphabet fell more than 1% on the news.

"Google’s monopolies in the publisher ad server and ad exchange markets...have enabled Google to introduce a series of anticompetitive policies, practices, and technology changes to its sell-side ad tech tools that were not in its publisher customers’ best interests," Brinkema wrote in her ruling.

"These changes decreased product quality and harmed competition by further entrenching Google as the dominant company in open-web display advertising. Google made these changes, despite customer complaints, by exploiting its durable monopoly power in the open-web display ad exchange and publisher ad server markets."

Advertising is Google's most important business segment, bringing in $72.4 billion of the company's $96.4 billion in total revenue in its most recent quarter. The company now faces the prospect of having to sell off parts of its advertising technology business, though it will have a chance to appeal Thursday's ruling.

Google previously lost an antitrust battle with the DOJ over its search business. That ruling could force Google to sell off parts of its search business including its Chrome browser.

Google's Big Tech rivals Amazon (AMZN) and Meta (META) are also each facing off in their own antitrust battles. Meta's trial got underway this week with CEO Mark Zuckerberg taking the stand to defend the company against claims that it used its power to buy or bury competing social media platforms.

Amazon, meanwhile, is waiting for its trial to begin. The FTC has accused the company of violating antitrust laws in the online retail space. 

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Email Daniel Howley at dhowley@yahoofinance.com. Follow him on Twitter at @DanielHowley.

Alexis Keenan is a legal reporter for Yahoo Finance. Follow Alexis on X @alexiskweed.

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