The two trains that were travelling on a single-track line between the towns of Bari and Barletta met with an accident, instantly killing 25 passengers.
Many more people are injured and some are in a critical condition. Local authorities have appealed for people to give blood.
It was unclear what led to the collision, which happened in good weather at 11:30 local time (09:30 GMT) in the southern region of Puglia.
Mr Renzi has ordered an official investigation in to the crash, which happened on Tuesday morning.
