The head of state met the U.S. secretary on the porch of the building of the Presidential Administration, an Ukrinform correspondent reports.
Poroshenko and Mattis will hold talks, and then they intend to make statements for journalists.
A visit by the U.S. secretary of defense to Ukraine, the first over the past decade, is made in accordance with previous agreements reached with the U.S. side during Poroshenko's visit to the United States in June this year. Mattis arrived in Ukraine on Wednesday evening.
Earlier, Ukrainian Defense Minister Stepan Poltorak said that the arrival of the Pentagon chief was a sign of U.S. support for Ukraine.