A U.S. federal judge has ruled that Google illegally built monopoly power in its online advertising business, siding with the Department of Justice in a landmark antitrust case that could force the tech giant to break up parts of its empire.

The decision, handed down on Thursday by Judge Leonie Brinkema in Virginia, found that Google’s control over both sides of the digital ad market — the tools for publishers and advertisers — gave it an unfair advantage that harmed rivals and consumers.

It marks the second major court win against Google in under a year and the third since late 2023, when a jury declared its app store to be an illegal monopoly.

At the heart of the ruling is Google’s $31 billion ad tech “stack,” a suite of tools used to match online ads with web publishers. The court found that Google tied together its ad server and publisher exchange in a way that locked out competitors and allowed it to “establish and protect” its dominance.

The DOJ argued that the setup created a structural conflict of interest, letting Google manipulate outcomes in its own favour — a view Judge Brinkema agreed with.

Google may now be required to divest part of its advertising business, though the company said it would appeal.

“We won half of this case and will appeal the other half,” said Lee-Anne Mulholland, Google’s Vice President of Regulatory Affairs. She noted the court had rejected claims tied to Google’s advertiser tools and acquisitions like DoubleClick.

The ruling adds to growing pressure on Google’s parent company, Alphabet, which is already facing regulatory scrutiny in the U.S. and Europe.

Critics have long warned that Google’s dominance in digital advertising raises prices for businesses and reduces visibility for smaller publishers. The Justice Department echoed that, stating Google’s behaviour “substantially harmed” publishers and the broader online information ecosystem.

The case also underscores a wider crackdown on Big Tech, as U.S. regulators ramp up efforts to rein in the power of companies like Meta, Amazon and Apple. Just this week, Meta CEO Mark Zuckerberg appeared in court over separate antitrust claims targeting the company’s acquisition practices.

While Google maintains that its tools benefit publishers, the court’s decision signals a new chapter in the government's antitrust fight — one that could alter how digital advertising works across the open web.

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