By Cara Lombardo, Liz Hoffman and Dana Cimilluca

Salesforce.com Inc.is in advanced talks to buy Slack Technologies Inc., according to people familiar with the matter, a deal that would unite a giant in business software with a buzzy newcomer on a mission to replace office email.

The companies could reach a deal within days, some of the people said. Slack, with a market value of more than $17 billion, would be Salesforce's largest acquisition ever. There's no guarantee the companies will reach an agreement.

Salesforce, which has a market value of some $230 billion, is the world's biggest seller of software that companies use to manage their customer relationships. Started 21 years ago and run by co-founder Marc Benioff, it was a pioneer in providing software via the cloud through a subscription rather than a costly upfront installation. Mr. Benioff, chairman and chief executive, is one of Silicon Valley's highest-profile executives, a status he recently burnished with the purchase of Time magazine.

The company has made a string of acquisitions to boost its reach, including a $15 billion-plus takeover of data-analytics platform Tableau Software last year and a smaller deal this year for cloud-software provider Vlocity Inc. It has also eyed social media, losing out on a deal for LinkedIn in 2016 and holding talks to buy Twitter Inc. in 2016 that were dropped after shareholders balked at the idea.

San Francisco-based Slack makes instant-messaging software that it pitches as a solution to what it says is cluttered and intrusive office email. It launched in 2009 as a gaming company called Tiny Speck, but pivoted to the messaging app it had developed for internal use. It is also trying to expand to become a platform that companies can use to tie other pieces of software together.

Once a niche product popular with technology developers, Slack has become more mainstream as the coronavirus pandemic sent millions of white-collar workers to home offices. But unlike with other remote-work companies whose shares have soared in recent months, such as Zoom Video Communications Inc., Slack's shares have risen only modestly. In June it withdrew its guidance for 2020 billings, citing uncertainty around the course of the virus.

Slack was one of the first high-profile companies to go public through an unusual method called a direct listing, in which the company simply floats its existing shares onto an exchange and lets the market determine the price without investment banks serving as gatekeepers.

Slack shares are little changed since their June 2019 debut on the New York Stock Exchange. The company's stock closed Tuesday at $29.58, compared with the reference price in the direct listing of $26.

The shares were up 36% Wednesday afternoon after The Wall Street Journal reported on a possible deal with Salesforce, whose stock dropped 2.6% to $254.05.

--Maureen Farrell contributed to this article.

Write to Cara Lombardo at cara.lombardo@wsj.com, Liz Hoffman at liz.hoffman@wsj.com and Dana Cimilluca at dana.cimilluca@wsj.com

(END) Dow Jones Newswires

11-25-20 1420ET