Halep, who was the year-end number one in 2017 and 2018, has been coached by Cahill for the past six years.
She won the 2018 French Open, her first grand slam title, under his guidance before Cahill left the team for a spell to spend more time with his family.
After six wonderful years working together, @darren_cahill and I have decided that it’s time to end our working relationship.— Simona Halep (@Simona_Halep) September 22, 2021
Thank you D for everything, for making me a better tennis player and a better person. 🤗🙏🏼 pic.twitter.com/VOEt4i81Bf
Cahill – who also guided Halep to the 2018 Australian Open final – returned to coach the Romanian ahead of the 2019 WTA Finals, following on from her success at Wimbledon.
The 29-year-old has endured an injury-hit 2021 and was knocked out of the US Open in the fourth round by Elina Svitolina.
In a post on her official Twitter account, Halep announced: "After six wonderful years working together, Darren Cahill and I have decided that it's time to end our working relationship.
"Thank you D for everything, for making me a better tennis player and a better person."