Sassoli, who was 65, had been admitted to hospital last month because of a "serious complication" related to his immune system, his office had said on Monday (January 10).
He had been president of the 705-seat parliament since July 2019. Sassoli, whose term in the predominantly ceremonial role had been due to end this month, had said he would not run for re-election.
"Today is a sad day for Europe. Today, our union loses a passionate European, a sincere democrat and a good man," von der Leyen said in a video statement in Brussels.