In the biggest real estate deal of the year, Google signed a contract on Thursday to buy one of the largest office buildings in Manhattan for more than $1.8 billion, according to two real estate executives who have been briefed on the deal.

Google put down a “substantial deposit” on the 15-story building at 111 Eighth Avenue, just north of the meatpacking district, and is expected to close the sale by the end of the year, said the executives, who spoke only on the condition of anonymity because they were not authorized to discuss the deal.

The contract not only signals a rebounding real estate market, at least for office buildings full of high-profile tenants paying good rents, but also reflects a vote of confidence by an expanding technology firm in New York City.

“This decision is further evidence that large-scale buildings are trading once again,” said Ross Moskowitz, a real estate lawyer at Stroock & Stroock & Lavan and a former city economic development official, “and high-end users are looking to take advantage of these opportunities.”