Slack has raised $427m in Series H funding, lifting its valuation to $7.1bn (£5.5bn).
The latest round was led by Dragoneer Investment Group and General Atlantic, and is in addition to the $841m the company has raised since its launch in 2013.
The workplace communications app currently claims to have eight million daily active users across 500,000 organisations, as well as three million paid users.
The latest funding comes after a $250m round last September took its valuation to $5.1bn.
In a blog posted on its website, Slack said: "We pursued this additional investment to give us even more resources and flexibility to better serve our customers, evolve our business, and take advantage of the massive opportunity in front of us."
The US-based company is becoming increasingly global, with more than half of its users now coming from outside of the US.
Italy is Slack's fastest growing market outside of America, while Japan has recently surpassed the UK as the firm's second largest market, based on the number of daily active users.
Slack has acquired three small US startups this year in Missions, HipChat and Stride.
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