The Celtic and Rangers managers are backing the Wee Box, Big Change lent appeal run by the Scottish Catholic International Aid Fund (Sciaf).
It got off to a flying start last week with the help of singer Susan Boyle and Paralympic gold-medallist rower David Smith.
The campaign, which is also supported by actor Robert Carlyle and broadcaster Kaye Adams, raised more than £830,000 for Sciaf's work last year.
Lennon said: "People often don't realise what a difference they can make by taking simple actions.
"With Sciaf in lent we can take action against poverty by giving up something like chocolate or crisps and popping our spare change in a wee box.
"This simple act can help people in developing countries across the world to build a better future for themselves."
McCoist, who will again fast for 24 hours, added: "When I back the campaign I know that I'm making a real difference to people living in extreme poverty across the world.
"Sciaf works alongside people in some of the world's poorest communities, so they can make a brighter future for themselves."