The 26-year-old, who has adapted his bowling action since suffering a stress fracture in his back in 2012, is especially effective in the shorter formats of the game and has played in Twenty20 franchise teams round the world.
"Marchant is a match-winner. He has pace, aggression – and he's the right age and at the right stage of his career to make an impact for us," said Croft.
"He will be a great addition to our squad."
De Lange, who played the last of his two Tests for South Africa in 2012 and his last one-day international in 2014, is in the "prime of his career", said Glamorgan's director of cricket Hugh Morris.
"We are delighted to have signed Marchant de Lange and to see him arrive in Wales today," said Morris.
Glamorgan open their second division County Championship campaign against Northamptonshire on Friday.