Turnbull was made head of Chinese equities at the firm in 2013, and he also served as an assistant fund manager on the Neptune Global Equity Fund.
In a statement on the departure, Neptune said Turnbull was leaving to “take up a role on an emerging market team”.
Neptune said the fund was being taken over by its head of emerging markets, Ewan Thompson, with immediate effect.
The China fund was taken over by Turnbull in 2009 and from then to date it has returned 137.5%, almost identical to the MSCI China index’s total return of 138.2%.
He previous managed the group’s Greater China Income and China Special Situations funds before they were closed as part of Neptune’s recent mass closures to concentrate its product range.
Robin Geffen, founder and CEO at Neptune, said: “Ewan has proven himself as one of the best fund managers in the IA Global Emerging Markets sector in recent years, as his performance record shows.
“He has been investing in China for almost a decade and worked closely with Doug for a number of years. We have every confidence that he will continue to deliver strong outperformance for our clients in the years ahead across both funds.”