Stewart Butterfield (C), co-founder and chief executive officer of Slack, and Allen Shim (2nd L), chief financial officer of Slack, ring the opening bell the New York Stock Exchange (NYSE), June 20, 2019 in New York City.
Drew Angerer | Getty Images
Salesforce shares dropped more than 4% on the news.
The deal would likely value Slack at more than its market cap of $17 billion, according to the report.
A deal is not guaranteed and the talks could call through, the Wall Street Journal reported.
Shares of Microsoft, which competes with Slack through its Teams offering, dipped slightly.
This is a developing story. Please check back for updates.