The Thor star has taken to his Twitter.com page to urge followers to join him in the challenge, which aims to raise awareness of poverty around the world.
Hiddleston tweeted, "(This week) I start living Below The Line to help Unicef stop children suffering from hunger. Take the challenge too. The World Bank defines poverty as living on less than £1 a day. The challenge is to buy all the food and drink you need to survive for five days with just £5 (£1 per day)."
The Brit added that he was going to "miss coffee very much" and shared photos of his first pared-down meals, including toast and a plate of rice and vegetables.