Local commerce offers services such as finding a discount from a nearby store, or a review of a neighborhood eatery, and the world's No. 1 search engine plans to compete in this market against Yelp and OpenTable.
The deal, for which Google did not provide financial information, gives it valuable content about restaurants, hotels and nightclubs that can be paired with its popular online maps and mobile search services.
Google needs to provide more than just directions to consumers seeking information about restaurants and other local businesses, said Marissa Mayer, Google's VP of Local, Maps and Location services, in an interview .
"It's also (about) getting them a sense of the place. A sense of what to expect," said Mayer. "Zagat reviews, in a few short lines and a few scores, gives you a great sense of a place very quickly when you're on the go."
The move is part of Google's push to adapt its online services for a world in which consumers increasingly access the Web on mobile phones such as Apple Inc's iPhone and rely on social networking services such as Facebook to get information from friends.
Last month, Google announced plans to acquire mobile phone manufacturer Motorola Mobility for $12.5 billion. The deal, if approved by regulators, will allow Google to produce its own line of smartphones based on its Android software.
"A reasonable person would say that Google may never beat apple in product design by itself. At least not for a sustainable period of time. But Google could better integrate content and have that become another reason to buy those devices," said Stifel Nicolaus analyst Jordan Rohan.
The 32-year-old Zagat, which polls consumers and compiles reviews about restaurants around the world, will become a cornerstone of Google's "local offering", Google said.
Google needs reviews and other content for its "Google Places" websites, in part to fend off criticism. It has been accused of using comments from review sites such as Yelp, essentially siphoning off their readers and, more importantly, their clicks. Google has toned down its borrowing of comments recently, Pitz said.
The Federal Trade Commission has been looking into the issue as part of a broad antitrust investigation, a source familiar with the probe has said.
The move raises the question of whether the search giant will start its own restaurant reservation service, building on existing ties with restaurants that advertise on it.
The shares of restaurant-booking service OpenTable, which also publishes reviews and ratings, closed down more than 8 percent at $57.50 on Thursday after hitting a low of 54.50 earlier in the day.
OpenTable is already reeling from financial results that have disappointed investors this year and the departure in May of CEO Jeffrey Jordan, who joined venture-capital firm Andreessen Horowitz. Jordan remains chairman.
Pitz said expanding into reservations would require extra steps such as building out reservation software and getting restaurants to install it, as well as building different relationships with the restaurants.
"It's apples and oranges," he said.
While much of Zagat's content is free and available to anyone, some content remains behind a paywall and it was unclear if Google would remove it.
Zagat gave Google a tongue-in-cheek rating on its home page on Thursday, awarding the Internet company a maximum 30-point rating for its "local, social, mobile and usefulness" categories. Industry analysts regard the local, social and mobile markets as some of the fastest-growing areas of the technology sector.
"We are thrilled to see our baby placed in such good hands and to start today as official 'Googlers,'" the founders said in a joint statement.
In late 2009 Google was in talks to acquire Yelp for at least $500 million, according to news reports at the time, but the deal fell apart.