NEW YORK — Wal-Mart has agreed to buy fast-growing online retail newcomer Jet.com, which had launched with a splash a year ago when it announced its intention to challenge online leader Amazon.

Wal-Mart is paying $3 billion in cash and another $300 million in stock.

The deal announced this morning underscores how serious Wal-Mart is about igniting its online business, which has been slowing even as it has been making big investments in new distribution centers and expanding services.

Buying Jet.com would let Wal-Mart compete more effectively with Amazon.com and other online retailers. The deal also reflects the difficulties for startups like Jet.com to make it on their own in a sphere dominated by Amazon.com with its network of distribution hubs and the powerful asset of its Prime membership program.

The acquisition is expected to close this year upon regulatory approval, Wal-Mart said.

Buying Jet.com will help Wal-Mart grab a higher-income customer who typically is younger. Jet.com has more than 400,000 new shoppers added monthly and an average of 25,000 daily processed orders. Wal-Mart says that it will also be incorporating some of Jet.com’s “smart” technology that lowers prices in real time by looking for ways to cut costs. For Jet.com, which has been pouring money into splashy TV ads and other marketing, it will provide big financial backing.

The move follows a series of acquisitions by major traditional retailers of online startups in an extremely competitive landscape. In January, Hudson’s Bay, which owns Saks Fifth Avenue, purchased flash-sales site Gilt Group. And in June, Bed, Bath & Beyond scooped up One Kings Lane.

“We’re looking for ways to lower prices, broaden our assortment and offer the simplest, easiest shopping experience because that’s what our customers want,” said Doug McMillon, president and CEO, in a statement.

Marc Lore, co-founder and CEO of Jet.com noted in a statement that the combination of Wal-Mart’s retail expertise, purchasing scale, sourcing capabilities, distribution footprint and digital assets together with the team, technology and business we have built at Jet — will allow the company to deliver more value to customers.