"I have sent a letter to President Putin with an appeal for immediate freedom for Nadiya Savchenko, including for medical reasons," Poroshenko said in a press conference after meeting Italian premier Matteo Renzi.
Savchenko, 33, has been on a hunger strike for 82 days to protest her arrest for alleged involvement in an attack that killed two Russian journalists on eastern Ukraine's frontlines, where she was enlisted as a volunteer in one of the battalions fighting pro-Russian rebels.
Kremlin spokesman Dmitry Peskov told Russian news agencies that the letter has not yet reached its destination and declined to provide any reaction.
Poroshenko on Monday honoured Savchenko, who was in October elected as a lawmaker and is a symbol of defiance in Ukraine, with the top state title Hero of Ukraine for her "indestructible will" and "sacrifice for the Ukrainian people.