The pair got together in Glasgow today to mark the start of the Scottish Catholic International Aid Fund's (Sciaf) Wee Box, Big Change appeal for Lent.
It asks people to give up one of their favourite treats for 40 days, such as chocolate, coffee or wine, and save the cash they would have used in a Sciaf Wee Box, to be donated at Easter.
The money will go towards the charity`s work in Africa, Asia and Latin America.
At the launch, Ms Boyle and Mr Smith were joined by Scotland`s most senior Catholic, Cardinal Keith O`Brien, and local schoolchildren.
Ms Boyle, who has publicly supported the campaign for three years, said she is giving up sweets, chocolate and crisps.
The singer, from Blackburn, West Lothian, 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.
"I'm supporting Sciaf's Wee box, Big Change campaign by giving up sweets, chocolate and crisps because I know Sciaf works side by side with people in poor countries to help them overcome poverty and the challenges they face.
"They help people to live their dreams. I'd encourage everyone to join the campaign."
Mr Smith travelled to Burundi in December to see the work Sciaf does there.
"With money donated by the Scottish public Sciaf is helping so many people become free from hunger and poverty by giving practical help such as agricultural training, seeds and tools," he said.
"When you see the difference Sciaf is making to the lives of extremely poor people every day, you have to help."
Other famous faces participating in the campaign include Rangers manager Ally McCoist, Trainspotting and Full Monty actor Robert Carlyle and broadcaster Kaye Adams.
Last year, more than £830,000 was raised to help people affected by hunger, poverty, war, disease and natural disasters in some of the poorest countries in the world, Sciaf said.
Actor Carlyle is giving up chocolate for Lent, which begins tomorrow.
"Sciaf is a great Scottish charity that's providing practical help to people in some of the poorest countries in the world - people affected by war, disease, poverty and natural disasters," he said.
"I'm going to play my part in Sciaf's Wee Box, Big Change campaign by giving up chocolate for Lent, so they can help more people in need. What will you do?"
Sciaf director Patricia Chale said: "We are extremely fortunate to have high-profile Scots including Susan Boyle, Robert Carlyle and David Smith, and thousands of Scots across the country taking part in the campaign.
"Their sacrifices during Lent will help Sciaf to support some of the world's poorest people to overcome hunger, poverty, war and disease."