Slack, the workplace chat app with an avid following in Silicon Valley, landed on Wall Street with a US$20bn valuation on Thursday, more than three times what it was worth when it last raised money only nine months ago.

The company's shares opened at US$38.50 and rose as high as US$42, marking a smooth start to trading for a rare direct listing on the New York Stock Exchange.

The first trade was confirmed shortly after midday in New York, after traders at Citadel Securities were able to find a clearing price for the initial buy and sell orders flooding to