The 36-year-old retired in 2010 after the previous year's Ashes series success against Australia, having played 79 tests, 141 one-day internationals and seven T20 matches. (Agencies)
He made 80 first-class appearances for Lancashire, and has been training with the county since the northern winter.
"I'm really honoured to be part of Lancashire once again," Flintoff said in a statement on the county's website (www.lccc.co.uk).
"It is something that I never thought would happen but after training with the squad over the last few months I am really happy that they have invited me to play. I have worked really hard to get back to my fittest and I hope that we have a successful summer. I'm just glad that I can be part of it."
Flintoff could play against Yorkshire at Old Trafford next Friday.
"We are delighted to have Fred involved once again at the club. He is Lancashire through and through and his record for both club and country speaks for itself," Lancashire cricket director Mike Watkinson said.
"Fred has been back at Emirates Old Trafford under his own steam since the winter and has been working with the Academy and in the nets with some of the other players," he said.
"Over a period of time he has picked up on his physical conditioning and this continues to improve. He has shown in practice that he still has plenty of class with bat and ball, and will be a great addition to the NatWest T20 Blast squad," he said.
The 36-year-old retired in 2010 after the previous year's Ashes series success against Australia, having played 79 tests, 141 one-day internationals and seven T20 matches.