American Express has considered dining perks one of the main benefits for its card members, helping them book tables at hot spots and offering them rewards points for doing so. Now it is taking a new step into the industry — by buying the country’s biggest privately held restaurant reservation service.

American Express announced on Wednesday that it had agreed to buy Resy, whose services allow customers to book reservations and restaurants to manage them.

It is part of the card company’s effort to expand in the hospitality industry, with earlier acquisitions like the travel-assistant app Mezi and the airport-lounge booking service LoungeBuddy.

“Putting Resy and American Express together will give Resy valuable scale,” Ben Leventhal, Resy’s co-founder and chief executive, said in a telephone interview.

Founded in 2014, Resy has 4,000 restaurants on its platform in 10 countries. The company has already made moves to consolidate the industry, having agreed to buy Reserve in November.

Yet it remains smaller than its best-known rival, OpenTable, which services 50,000 restaurants worldwide.

Resy and American Express had already been partners, but the card giant decided several months ago that buying Resy made sense, said Chris Cracchiolo, American Express’s senior vice president for global loyalty and benefits.

“American Express wants to be more central in our customers’ everyday lives,” he said in an interview.

American Express is hoping that the deal works out better than Booking Holdings’ $2.6 billion takeover of OpenTable in 2014 — which was followed by a $941 million write-down of the acquisition two years later.

Mr. Cracchiolo declined to comment on the OpenTable deal, but said American Express was committed to helping Resy expand.

Financial terms of the transaction were not disclosed. Resy will continue to be led by Mr. Leventhal, and, at least in the short term, its existing offerings will remain unchanged.