A US judge rules Google monopolised ad markets—paving the way for a government-enforced breakup.

Google’s grip on the digital advertising world just took a massive hit. A federal judge has ruled that the tech giant illegally monopolised key ad markets, clearing the path for US regulators to demand the breakup of its advertising empire.

The landmark decision, handed down on Thursday by Judge Leonie Brinkema in Alexandria, Virginia, found Google guilty of “willfully acquiring and maintaining monopoly power” over two crucial components of the online ad ecosystem: publisher ad servers and ad exchanges.

These tools sit at the heart of the digital ad market. Publisher ad servers help websites manage their ad inventory, while ad exchanges facilitate real-time buying and selling between advertisers and publishers. Prosecutors said Google used these tools not to compete, but to dominate—locking in customers, acquiring rivals, and bending the rules in its favour.

In her ruling, Judge Brinkema agreed. She backed the US Department of Justice and a coalition of state attorneys general, who argued that Google crushed competition through acquisitions and restrictive business practices. However, the judge dismissed a separate claim that the company monopolised advertiser ad networks.

The ruling now triggers the next phase: a hearing to decide how to restore competition. The government has already signalled its intent—urging the court to force Google to sell off its Google Ad Manager, which bundles both the publisher server and ad exchange platforms. A timeline for that hearing has yet to be announced.

Google has vowed to fight back.

“We won half of this case and we will appeal the other half,” said Lee-Anne Mulholland, the company’s vice-president of regulatory affairs. “Publishers have many options and they choose Google because our tools are simple, affordable and effective.”

But Google’s legal troubles are far from over. This is just one of two looming battles the company faces. In Washington next week, another federal judge will preside over a separate case in which the Justice Department is asking for the forced divestment of Google’s Chrome browser and a shake-up of its dominance in online search.

The company is also no stranger to scrutiny from overseas regulators. In Europe, Google has reportedly considered selling off parts of its ad business to appease antitrust watchdogs, though no formal agreement has been made.

The Virginia case stemmed from a three-week trial held last year, where prosecutors laid out Google’s strategy for domination. It wasn’t just innovation, they said—it was textbook monopoly behaviour: acquiring rivals, setting barriers to entry, and stifling interoperability with other ad platforms.

Google countered that the claims were based on outdated practices, arguing its tools had since become more open and compatible with competitors. The company also said prosecutors had downplayed fierce competition from Amazon and Comcast, particularly as advertisers shifted to apps and streaming services.

The judge wasn’t convinced.

Her ruling not only sides with the government but also adds to mounting legal momentum against one of the world’s most powerful companies. With the court now weighing forced divestitures, Google could face the most significant regulatory shake-up in its history.

If prosecutors succeed, it could reshape the digital advertising industry overnight—breaking apart a system that, for over a decade, has been dominated by one name.