Six were killed and five others injured in June this year after a truck hit a passenger car, causing a pile-up involving seven vehicles. Some of the vehicles caught fire.
Ukrainian national Andrey Sirovatskyi, who witnessed the crash, rushed to the burning cars and saved the lives of three people.
Sirovatskyi, who is a truck driver by profession, applied for Polish citizenship for himself and his family in July.
They all received Polish citizenship at a ceremony on Wednesday.
“Seldom does one acquire Polish citizenship in three-and-a-half months. I am happy that today, on behalf of President Andrzej Duda, I am granting you Polish citizenship,” Władysław Dajczak, the governor of the Zachodniopomorskie province in north-western Poland, said at the ceremony.
Around 1.3 million Ukrainian citizens work in Poland, the Gazeta Wyborcza daily reported earlier this year.