Mr Renzi, 42, won more than 70% of the vote in Sunday's primary among party supporters, leaving two other contenders far behind.
The victory propels him back to the forefront of national politics.
Mr Renzi resigned as prime minister and party leader after his constitutional reform plan was rejected in a national referendum in December.
"This is an extraordinary responsibility. Heartfelt thanks to the women and men who believe in Italy," he wrote on Instagram on Sunday.
