Billionaire investor Icahn last week purchased a 4.3% stake in Yahoo! and declared his intent to replace the company's 10-strong board if it did not reopen takeover talks with Microsoft.
His move has prompted other investors to buy Yahoo! shares in order to intensify the pressure on the company, which is led by founder and chief executive Jerry Yang.
Yesterday, Pickens, an oilman, investor, 80s corporate raider and head of hedge fund BP Capital Management, told business news channel CNBC he had bought 10m Yahoo! shares, giving him a stake of around 0.75% at a rough cost of $250m.
Pickens said he did not know much about the company but pledged his support for Icahn: "Carl's a smart guy. He does great things for shareholders. It is being served up to Microsoft now. Are they going to step up and take it and go on down the road? I don't know what they're waiting for."
A hedge fund called Third Point has bought a stake of more than 5m shares and is also behind Icahn.
Last week another hedge fund, Paulson & Co, said it was putting its recently acquired 50m shares behind Icahn's efforts.
However, Microsoft has moved on since Yahoo! rejected its last $33 per share offer, which valued its target at $44.6bn (£22.6bn).
At the weekend Microsoft changed its position and said it was in talks with Yahoo! about a limited transaction, believed to be either an advertising partnership or an acquisition of Yahoo!'s search business.
Yahoo!'s share price closed yesterday at $27.48, down 0.72%.