Roy Keane has been appointed assistant manager at Aston Villa.
Villa boss Paul Lambert approached Keane at the end of last season with a view to combining the role with his existing job as number two to Republic of Ireland manager Martin O'Neill.
O'Neill has given the move his approval and Keane, 42, will be in place when pre-season training begins on Monday.
"I am delighted to be joining Villa and look forward to what promises to be an exciting challenge," Keane said.
The club did not disclose the length of Keane's contract.
"I am grateful to both Villa and the Football Association of Ireland for making this happen and would like also to thank Martin O'Neill," Keane added.
"I am relishing the prospect of linking up with Paul Lambert and combining both roles to the full advantage of both Villa and Ireland."
Former Manchester United midfielder Keane, who has managed Sunderland and Ipswich, replaces Lambert's previous assistant Ian Culverhouse, who was dismissed in May following an internal investigation.
"Roy has a great knowledge of football, on the coaching side as well, and he'll be a great asset and a big help to everybody here at Aston Villa Football Club," Lambert said.
"I'm really looking forward to working with him and I'm delighted that he's here. The players will love working with him.
"He has worked under some great managers, some of the most successful in the game, and he'll be a great help to myself as well."
Keane, who was recently linked with the managerial vacancy at Celtic, was appointed along with O'Neill by the Republic on a two-year deal in November.
Villa finished 15th in the Premier League last season, only five points above the relegation places.
BBC Radio 5 live's Pat Murphy said: "Lambert needs a strong assistant to sort out several high-earning, under-achieving Villa players. Keane is ideal for that."
Villa begin the new Premier League season at Stoke City on the weekend of 16-17 August.
Keane joins Villa amid efforts by owner Randy Lerner to sell the club.
Meanwhile, Villa have confirmed midfielder Gary Gardner, 22, and striker Callum Robinson, 19, have signed new two-year deals.