The Trainspotting star will hop onboard a yacht with British sailor Alex Thomson for the 50-mile (80-kilometre) high speed sprint around England's Isle of Wight.
The first place team will take home $16,000 (£10,000) for their chosen charity - and McGregor has vowed to donate the cash to the food crisis in East Africa if he wins.
He says, ""I'm excited to experience the thrill of yacht racing from the very deck of the boat and I'll be doing my best to help Alex win so that we can donate the prize money to UNICEF's East Africa Appeal.
""There's never been a more important time to support the children of the region who so desperately need our help.""
Queen Elizabeth II's granddaughter, newlywed Zara Phillips, is also taking part in the annual Artemis Challenge on Wednesday (10Aug11).