Shakhtar's executive manager Serhiy Palkin posted a photograph of Marlos holding his Ukrainian passport on Facebook.
"It was his own decision. This is quite good news for Shakhtar as we now have one more Ukrainian player. It is also good for the Ukraine team," Shakhtar coach Paolo Fonseca told a news conference on Friday.
"Marlos is a quality player with outstanding personal qualities. I think he will be of great help for the Ukrainian national team."
Local media expect Ukraine coach Andriy Shevchenko to call up Marlos for two decisive World Cup qualifiers away to Kosovo and at home to Croatia on Oct. 6 and 9 respectively.
Two wins would guarantee Ukraine second spot in Group I and a likely playoff to qualify for the 2018 finals in Russia.
Marlos came to Ukraine in 2012 to join Metalist Kharkiv and two years later moved to Shakhtar, scoring 34 goals in 180 games.
(Editing by Ken Ferris)
