OpenTable is getting into the food delivery business.

The world's biggest online restaurant reservation service — which was founded 21 years ago — has been watching warily as more and more diners opt for delivery. Between May 2018 and May 2019, U.S. restaurant visits were flat at 23.8 billion, but deliveries rose 3% to 2 billion, according to NPD Group, a market research company.

So OpenTable decided to partner with three delivery companies — Caviar, GrubHub and Uber Eats — to offer that service through its app.

"We want OpenTable to be the go-to dining app for every meal occasion," said Steve Hafner, who leads OpenTable and Kayak, an airline fare search engine. Both companies are owned by travel conglomerate Booking Holdings.

When OpenTable's updated site launches this week, it will give diners a delivery option for 8,000 restaurants in 90 U.S. cities. If they select delivery, users will be directed to the restaurant's preferred service to complete the transaction. If a restaurant works with more than one delivery company, each option will be shown. Eventually, OpenTable wants to post estimated delivery times and costs for each service as well.

Hafner said OpenTable decided to add a delivery option about nine months ago. Partnering was the easiest and fastest way to get into the business, Hafner said.

According to Technomic, the top five food delivery companies in the U.S. had $13.5 billion in sales between May 2018 and May 2019.

OpenTable had to get into delivery because it's the only part of the restaurant business that's growing, said David Portalatin, a vice president and food analyst at NPD.