The 31-year-old has signed a one-year contract at Tannadice.
Flood, who was signed by Aberdeen three years ago and still had 12 months remaining on his contract at Pittodrie, was released by the club on Thursday afternoon.
He was previously on loan with United from Cardiff two seasons in succession from July 2007, and following spells with Celtic and Middlesbrough he returned to Tayside in 2011 for two years.
Flood revealed he opted to make the switch after new manager Ray McKinnon outlined his plans for the club.
"The reason I have come back to United is primarily the manager. Everything he told me of his plans for the club I liked. His ambition, drive and desire convinced me this move would be good for me," Flood said.
"I'm the same player, I want my team-mates to do well and I will demand that of them and I will be a good team-mate back.
"It is difficult for me to say how tough the Championship will be because I have played in the Premiership but I would not have dropped down the divisions for any other club and I hope that shows how much I want to be back at United.
"My drive is to play every week and help the younger players develop good habits in training and match days. I am sure the manager has installed that already but I am happy as a senior pro to also add my input and assist the team in any way I can."
Following Flood's exit from Pittodrie, Aberdeen boss Derek McInnes was full of praise for the Irishman.
McInnes said: "Everyone at Aberdeen FC wishes Willo well. He was as significant a signing that I've made since I came here and has played a pivotal role both on and off the park and has been a pleasure to work with.
"I really appreciated his demanding nature and he's a very popular figure around the place.
"We wish Willo all the very best with whatever club he continues his career at and he'll always be welcome back at Pittodrie in the future."
Flood was a favourite of Aberdeen-Mad's, and we wish him all the best in the future.