The former Llanelli and Rotherham star made 59 Leinster appearances between 2004-'07 (including 14 in the Heineken Cup) and will be eligible to compete in both the Magners League and Heineken Cup competitions.
Leinster coach Michael Cheika commented: "We are delighted that Guy has agreed to sign a short-term contract with Leinster. Guy has huge experience in the game and was a popular member of the squad up until his retirement and I have no doubt he will give us the depth in the scrum-half position that we need at this important part of our year. He is a proven leader and it is also an added boost that he is familiar with the set-up.
"I would also like to express my thanks to Otley for their understanding and co-operation in facilitating the move."
Name: Guy Easterby
Position: Scrum Half
Club: Blackrock College RFC
School: Ampleforth College
Date of Birth: 21/03/1971
Official Leinster Caps: 59
Points Scored: 25 (5 tries)
Leinster 'A' Caps: 2 (v Ulster, Munster)
Magners/Celtic League/Cup Caps: 45 (5 tries)
Heineken Cup Caps: 14
Senior Debut: Celtic League, 17-15 win v Munster at Donnybrook, 18.09.2004
Heineken Cup Debut: 30-11 win v Bath at Lansdowne Road, 30.10.2004
Ireland Caps: 28
Ireland Points: 30 (6 tries)
Ireland "A" Caps: 5