The investment is being led by SoftBank Group Corp and venture capital firm Accel, a previous investor, said the people, who declined to be identified. SoftBank has been heavily investing in startups, and it recently launched a $93 billion Vision Fund for technology investments.
Other previous investors are participating. Slack has in the past raised money from venture firms GGV Capital, Spark Capital and Thrive Capital, among others.
Slack spokeswoman Julia Blystone declined to comment on "funding rumors". Slack is headquartered in San Francisco.
Slack's sizeable funding round reflects the trend of a growing number of $100 million-plus checks pouring into technology startups. In the second quarter this year, there were 34 venture capital deals of $100 million or more, nearly triple the 12 such transactions in the first quarter, according to data firm PitchBook Inc.
These large rounds have helped drive an uptick in startup funding since the end of last year, with venture capitalists investing $15.7 billion in companies during the second quarter this year, a 27 per cent increase from the first quarter and making for the strongest quarter in a year, according to Thomson Reuters data.