The 35-year-old has penned a two-year deal following his exit from Blackburn Rovers.
Mulgrew, who has 44 Scotland caps, has extensive experience both in Scotland and England.
He spent two seasons with Aberdeen before joining Celtic – the club where he started – in 2010. At Parkhead he won five league titles and two Scottish Cups.
During his first spell at Celtic he had a loan stint at Dundee United during the 2005/06 season, playing 14 times.
Mulgrew is new United boss Tam Courts’ first signing.
"I’m excited to have a player of Charlie’s winning pedigree available to me for the next two years," said Tam. "I believe that he joins an existing group of senior players who already give everything to make Dundee United better on the pitch.
"Charlie is renowned for his leadership and impact on young players which is something that will be valuable in their development."