Senior, 37, played alongside head coach Mark Aston in Sheffield’s famous 1998 Challenge Cup final victory over Wigan, and he will combine the new role with his day job at the Leeds Rugby Foundation.
He will alongside current assistant coach Dave Larder, being involved during the week in training sessions and on game days.
Senior said: “It’s great to be heading back to the Eagles. I have a lot of fond memories at the club and I’m excited about getting involved.
“I’ve been interested in coaching for some time now and see it as the next natural progression after finishing my playing career.
“Mark (Aston) has got a really good, dedicated squad of players and I hope I can play my part and help the club retain the Championship for a third successive year. I can’t wait to get started.”
Senior moved on to Leeds in 1999, and became the first player to reach 500 appearances in Super League, scoring 226 tries up to his retirement in 2011.
He won four Super League Grand Finals with the Rhinos, and was named in the Dream Team five times.
Aston added: “I’m thrilled Keith is coming back to us. He has a lot of knowledge around the game and has some an abundance of experience on the big stage with numerous Challenge Cup and Grand Final appearances under his belt along with a raft of international appearances.
“We made history this year becoming the first team to become back-to-back Champions and we are aiming to win it again in 2014.”