The man who led to the pro-union Better Together campaign during the Scottish independence referendum insisted the Glasgow MSP is the best candidate to take on the SNP and Tories north of the border.
Mr Sarwar is up against left-winger Richard Leonard in the contest to replace Kezia Dugdale as Labour leader.
Mr Darling said Mr Sarwar’s support for European single market membership shows that he is best placed to challenge the Tories over Brexit.
“Our next leader needs to be able to take on both the Tories and the SNP, both of whom will be fighting to be the biggest party in the Scottish Parliament," Mr Darling said.
“To beat them both we need a leader who is unequivocally for staying in the UK and unequivocally pro-European. There is no doubt where Anas stands on both these crucial issues.“Anas led the Labour campaign for a No vote in 2014. The fight against nationalism was tough. Not everyone would have shown the leadership Anas did, but he stepped up to lead an energetic campaign, visiting ever part of Scotland. Anas showed he can take on the SNP and win."
The ex- Scottish Secretary said Mr Sarwar's support for single market membership is "essential for jobs."
“That’s why I’m supporting him, not just as next leader, but as the next Labour First Minister of Scotland,” he said.
Mr Leonard already has the support of most unions in the race to be next Labour leader and is seen as the "Corbynista" candidate, but Mr Sarwar has the backing of most Labour MSPs