The ruling is a big setback for the government, which wrested PrivatBank from Kolomoisky, a co-founder of the bank, in December 2016.
PrivatBank was nationalized as part of a clean-up of the banking system backed by the International Monetary Fund, which supports Ukraine with a $3.9 billion loan program.
Ukraine’s dollar-denominated Ukraine bonds fell more than 1 percent after the ruling by a Kiev court as President Petro Poroshenko said in a televised address that overturning nationalization threatened “default and a new economic crisis.”
He has previously said that any backsliding on PrivatBank would spark a “deep crisis in relations with the IMF.”
The central bank said it was impossible to reverse the nationalization and it would appeal against the ruling.
After taking over the bank the government had said it wanted to recover money it says was siphoned out while Kolomoisky owned it. It has shored up the lender with billions of dollars since it was nationalized.
Kolomoisky denies any wrongdoing and says the bank was forcibly nationalized without proper justification.