Weah, 51, said he had been given an "overwhelming" mandate to end corruption in Liberia and said he wanted to narrow the gap between the rich and poor.
Growing up in the slums of Monrovia to become one of Africa's greatest footballers, Weah played for top-flight European teams including AC Milan and Paris St Germain.
He succeeds Ellen Johnson Sirleaf, Liberia and Africa's first female elected head of state, who took power 12 years ago. She is credited with shoring up peace after a bloody civil conflict, but criticized for failing to tackle elite graft or do much to lift Liberians out of poverty.
Founded by freed American slaves, Liberia is Africa's oldest modern republic. But it has been plagued by years of unrest and bloodshed, including two civil wars in the 1980s and 90s that were notorious for their brutality and use of child soldiers.