Acting US Ambassador to Ukraine Bill Taylor, whose testimony helped build the case for impeaching Donald Trump, is to leave his post next month.
Mr Taylor will step down in January when his temporary appointment is due to expire, the New York Times reported.
His tenure could have been extended by the Republican-controlled Senate but no such move has been made.
Mr Taylor took up the post in June after Mr Trump controversially withdrew the then ambassador, Marie Yovanovitch.
Ms Yovanovitch has said she was the victim of a smear campaign by Mr Trump's personal lawyer, Rudy Giuliani.
A replacement for Mr Taylor has not yet been announced.
News of his departure was met with concern by Senator Robert Menendez, the most senior Democrat on the Senate Foreign Relations Committee.
He said he feared it would create a vacuum in Ukraine, which is still grappling with Russian-backed separatists.
In his testimony before the impeachment inquiry last month, Mr Taylor said that a member of his staff overheard a telephone call in which President Trump inquired about “the investigations” into Mr Biden.
The call was with Gordon Sondland, US ambassador to the European Union, who reportedly told the president over the phone from a restaurant in Kyiv that “the Ukrainians were ready to move forward”.
After the call, the staff member “asked Mr Sondland what President Trump thought about Ukraine”, Mr Taylor said.
Mr Taylor said: “Ambassador Sondland responded that President Trump cares more about the investigations of Biden.”
President Trump is accused of withholding US military aid to Ukraine in order to pressure the country's president to publicly announce a corruption inquiry into Mr Biden. He denies any wrongdoing and has described the impeachment inquiry as a “witch hunt”.
A majority of House Democrats are expected to vote to impeach Mr Trump on Wednesday.