Grateful for his support, Sciaf director Patricia Chale said: "I've no doubt that Sir Alex's public support will encourage many more people to find out about the campaign, get their own Sciaf wee box, and make a donation. Every penny goes to helping people in urgent need."
The football manager joins other big names in supporting the campaign, such as singer Susan Boyle, actor Robert Carlyle, football managers Neil Lennon and Ally McCoist and broadcaster Kaye Adams.
Boyle said: "It's important that we all have dreams. I'm very fortunate to be living mine. For other people affected by terrible poverty, war or disease, they dream of peace, good health and having enough food to eat."
This year's Wee Box campaign runs until Friday March 29 and features Sciaf's work in the African country of Burundi where many people have suffered from war.