Simona Halep will work with Patrick Mouratoglou, the long-time coach of Serena Williams, on a full-time basis.
American great Williams has been coached by Mouratoglou since 2012 but has not played since injuring her leg at Wimbledon in June.
Mouratoglou said he had missed coaching over the past eight months.
"I had a conversation with Serena, and the door opened for me, at least short term, to work with someone else," he wrote on Twitter.
Former world number one Halep beat Williams to win Wimbledon in 2019.
She won her maiden Grand Slam title at the French Open the year before but has struggled in the past 12 months with injury and is now 20th in the rankings.