Nick Hulme has resigned as chief executive of Croydon University Hospital.
He had been on unexplained leave since January 30 with questions over his future.
Today, he announced his resignation, with deputy chief executive Tony Leonard taking over while interim arrangements are finalised.
A spokeswoman from Croydon Health Services NHS Trust Chief Executive said: "On behalf of the Board we would like to express our thanks to Nick for his contribution to the Trust, in particular the role he played in leading it through its successful integration with Croydon Community Services, his focus on improving quality, safety and patient care, and the delivery of sound financial and organisational performance.
"The next twelve months remain an important time for the Trust as we continue to focus on providing high quality care to the people of Croydon and securing our future by becoming a Foundation Trust."
The statement gave no explanation as to why Mr Hulme was resigning, or the reason for his leave.