Any deal would value Slack above its current $17 billion market capitalization, The Wall Street Journal reported. It would represent Salesforce's largest acquisition to date.
There is no guarantee that the acquisition talks will lead to a deal and Salesforce could ultimately choose a different target, sources told the Journal.
Slack jumped 7.1% to $31.68 on the news before being halted due to volatility. Salesforce shares dropped more than 3% on the news to $252.70, though the stock has risen about 55% year to date.
Requests for comment from both Slack and Salesforce weren't returned immediately Wednesday.
Analysts at Wedbush published an industry note Wednesday afternoon following the news, saying "This would be an aggressive move for Benioff & Co. to further expand its cloud enterprise product offering."
"This deal would be a major shot across the bow at Microsoft with the company's Teams offering a direct messaging competitor against Slack that has been a clear hurdle to growth and now would be a two horse race between Microsoft and Salesforce," analyst Dan Ives wrote.
Earlier this month, Saleforce was downgraded to equal-weight from overweight by analysts at Morgan Stanley on valuation concerns.
"At CRM's current growth, scale and market cap, an increasing focus on [free cash flow] and earnings is likely necessary for further price appreciation," said Morgan analyst Keith Weiss.
"However, subscription model dynamics and management's growth philosophy may make that difficult near-term, pushing us to an [equal-weight] rating."
Salesforce is scheduled to report earnings on Dec. 1, with analysts estimating earnings of 75 cents a share on revenue of $5.25 billion.
Salesforce was added to the Dow Jones Industrial Average in late August.
At the time, Morningstar analyst Dan Romanoff said "we believe Salesforce.com represents one of best long-term growth stories in software. Salesforce.com has assembled a front-office empire that it can build on for years to come.”