Filed in federal court in Virginia, the case alleges that Google abuses monopoly power in the ad-tech industry, hurting web publishers and advertisers that try to use competing products. Eight states, including California and New York, joined the Justice Department’s lawsuit.

The lawsuit asks the court to unwind Google’s “anticompetitive acquisitions,” such as its 2008 purchase of ad-serving company DoubleClick, and calls for the divestiture of its ad exchange.

“For 15 years Google has pursued a course of anticompetitive conduct that has allowed it to halt the rise of rival technologies, manipulate auction mechanics, insulate itself from competition, and forced advertisers and publishers to use its tools,” Attorney General

Merrick Garland

said at a press conference Tuesday. “Google has engaged in exclusionary conduct that has severely weakened if not destroyed competition in the ad-tech industry.”

A Google spokesman said the lawsuit “attempts to pick winners and losers in the highly competitive advertising technology sector.”

“DOJ is doubling down on a flawed argument that would slow innovation, raise advertising fees, and make it harder for thousands of small businesses and publishers to grow,” the spokesman said.

By calling for specific divestitures from Google’s ad-tech business, the Justice Department lawsuit went further in seeking a breakup than some antitrust experts had expected. Shares of Alphabet fell by about 2% in trading on Tuesday.

Though largely invisible to internet users, the ad-tech tools controlled by Google facilitate much of the buying and selling of digital ads that helps fund online publishers. Google’s business includes a tool publishers can use to offer ad space, a product for advertisers to buy those slots and an exchange that automatically links bidders with webpages as they are being loaded for individual users.

Big tech companies such as Google are under a barrage from lawmakers and regulators across multiple continents who have targeted the companies’ dominance in online markets. Justice Department officials also are investigating

Apple Inc.

The Federal Trade Commission has sued

Meta Platforms Inc.’s

Facebook unit over antitrust allegations and

Microsoft Corp.

to block its planned $75 billion acquisition of

Activision Blizzard Inc.

President Biden recently urged lawmakers from both parties to unite behind legislation seeking to rein in tech giants. The European Union also has opened cases looking at alleged anticompetitive conduct by Google, Meta and other companies.

The Justice Department’s 2020 lawsuit against Google targeted its position in online search markets, including an agreement to make Google search the default in Apple’s Safari web browser. Google is fighting the case, which is expected to go to trial this year.

Alphabet gets about 80% of its business from advertising. The Justice Department’s new suit targets the subset of that ad business that brokers the buying and selling of ads on other websites and apps. Google reported $31.7 billion in revenue in 2021 from that ad-brokering activity, or about 12% of Alphabet’s total revenue. Google distributes about 70% of that revenue to web publishers and developers.

Last year, Google offered to split off parts of its ad-tech business into a separate company under the Alphabet umbrella to fend off the most recent Justice Department investigation. DOJ officials rejected the offer and decided to pursue the lawsuit instead.

For years, Google has faced allegations from advertising- and media-industry executives, lawmakers and regulators that its presence at multiple points of the online ad-buying process harms publishers and gives it an unfair advantage over rivals. Google also operates the most popular search engine and the largest online video-streaming site, YouTube, giving rise to allegations it has tilted the market in its own favor.

Rivals say that Google’s power in digital advertising stems from a series of acquisitions Google used to build its ad-tech business, beginning with the company’s $3.1 billion purchase of DoubleClick. The FTC approved the merger in a controversial decision. Google went on to purchase a host of other startups including the mobile-advertising company AdMob.

“Having inserted itself into all aspects of the digital advertising marketplace, Google has used anticompetitive, exclusionary, and unlawful means to eliminate or severely diminish any threat to its dominance over digital advertising technologies,” the complaint read.

Google has said it has no plans to sell or exit the ad-tech business. It has also strongly contested claims in a lawsuit filed by state attorneys general, led by Texas, containing allegations similar to the Justice Department complaint. A federal judge denied the bulk of Google’s motion to dismiss the case last year, allowing it to proceed to the discovery stage and ultimately toward trial.

Any divestiture of parts of Google’s ad-tech business would cause big ripple effects across the online advertising industry, which has recently shown signs of weakness as consumers dial back purchases in response to worsening economic conditions.

Breaking off parts of Google’s ad-tech business from the rest of the company could take years of litigation to resolve. Depending on the outcome of the case, ad-tech executives have said the results could range from a higher share of ad dollars flowing to publishers to lower overall spending because digital ads would be less efficient without Google brokering them.

The 149-page complaint makes detailed allegations about the internal workings of Google’s ad-tech operations. The suit alleges, for instance, that Google used anticompetitive tactics to build up the market share of its own ad server, which issues requests for advertisements on behalf of websites, and then used that market power to effectively push publishers into sending their ad inventory only to Google’s in-house ad exchange, AdX.

The Justice Department argues, in part, that this conduct locked out rival ad-tech providers, increasing prices for advertisers and costs of publishers.

“Google keeps at least thirty cents—and sometimes far more—of each advertising dollar flowing from advertisers to website publishers through Google’s ad tech tools,” the lawsuit alleges. “Google’s own internal documents concede that Google would earn far less in a competitive market.”

The lawsuit also alleges that Google executives worked to kill a rival online-bidding technology called “header bidding,” which the lawsuit says the company referred to internally as an “existential threat.” As part of a plan dubbed Project Poirot, the company allegedly changed its own ad-buying tools to underbid on behalf of advertisers when they turned to outside ad exchanges that used header bidding, so those rivals would lose more auctions and “dry out,” the complaint says.

At one point, Google also approached

Amazon.com Inc.,

to ask “what it would take for Amazon to stop investing in its header bidding product,” the complaint alleges, adding that Amazon rebuffed those requests.

“Google uses its dominion over digital advertising technology to funnel more transactions to its own ad tech products where it extracts inflated fees to line its own pockets at the expense of the advertisers and publishers it purportedly serves,” the complaint read.

The Justice Department case overlaps in some ways with the late 2020 lawsuit from the group of U.S. states led by Texas.

In Tuesday’s complaint, the Justice Department quotes some of the same internal communications as the Texas-led lawsuit, including how one Google executive compared the company’s control over ad-tech to the financial sector: “The analogy would be if Goldman or Citibank owned the NYSE,” referring to the New York Stock Exchange.

The case also shares similarities with an investigation that the EU’s top antitrust enforcer, the European Commission, opened in 2021, as well as one by the U.K.’s Competition and Markets Authority. Those probes are exploring allegations that Google favors its own ad-buying tools in the advertising auctions it runs, but also look at other elements of Google’s ad-tech business. The EU, for instance, is also looking at Google’s alleged exclusion of competitors from brokering ad-buys on its video site YouTube.

Mr. Garland said Tuesday that the Justice Department filed its own lawsuit because the federal government was harmed by Google’s allegedly monopolistic conduct. Federal agencies have since 2019 spent over $100 million on display ads, the complaint says. The government paid inflated fees and was harmed by manipulated advertising prices because of Google’s anticompetitive conduct, the lawsuit alleges.

Jonathan Kanter,

the assistant attorney general for antitrust, said while there are similarities with other lawsuits against Google, the Justice Department’s complaint is based on its own investigation that yielded “meticulous detail” about Google’s ad-tech business.

“We detail many facts, many episodes that in the individual and in the aggregate have maintained numerous monopolies,” Mr. Kanter said.

Google has attempted to settle the claims against its ad-tech business. In addition to offering to split off parts of its ad-tech business to avoid the Justice Department suit, the company last year discussed with the EU an offer to allow competitors to broker the sale of ads directly on the video service.

In 2021, the company agreed to give U.K. antitrust regulators effective veto power over elements of its plans to remove a technology called third-party cookies from its Chrome browser to settle an investigation there into the plan.

In France, Google agreed to pay a fine of 220 million euros, equivalent to about $239 million, and to improve data access to competing ad-tech companies, to not use its data in ways rivals couldn’t reproduce to settle a similar antitrust investigation in the country.

—Keach Hagey contributed to this article.

Write to Miles Kruppa at miles.kruppa@wsj.com and Sam Schechner at Sam.Schechner@wsj.com