The broadcaster told the stock exchange his resignation was effective immediately and group financial controller Graeme Cassells would act in the role.
Koppelman was due to present Nine’s full-year results on August 27 with chief executive Hugh Marks.
Nine said preparations for these results were well progressed.
Nine expects earnings before interest, tax, depreciation and amortisation to be in the range of $390 million to $410 million.
In March, Nine withdrew its profit guidance as the advertising market declined amid the coronavirus pandemic.
The broadcaster has since slashed costs and offloaded its interest in Australian Associated Press and New Zealand publisher Stuff.
Steven Deare in Sydney