Trentin took gold after a four-man burst to the line, outsprinting Dutch rider Mathieu van der Poel, and Belgian winner Wout van Aert after five hours 50 minutes of hard graft in the rain over 230.4km.
"It's incredible, really incredible to have won after everything I've been through recently," Trentin said. "I want to thank everyone who was there for me. This is a great day!"
"We wanted to be always in the front because we didn't want to take any risks. The race was made even harder by the weather, but we managed to deal with it the best we could.”
“There was a moment of worry a few kilometres from the end when there was a crash within the leading group, but fortunately it didn’t affect me, and I turned around and saw that Davide Cimolai (Groupama-FDJ/Italy) also managed to stay upright.
“Before that crash, we decided we were going to attack and afterwards we had to evaluate how to act and on reflection the crash probably did us a little favour. I asked Cimolai to lead me out and he did. He did a spectacular job, as did all the Italian team."
World champion and favourite Peter Sagan who suffered with injury last month en route to winning a sixth green jersey at the Tour de France, was two minutes 45 seconds behind the leaders when he abandoned the race on the 11th of 16 laps. The Slovakian was also earlier forced to chase back on after a lengthy wheel-change.
"I was not in good shape and I still have a lot of pain from my crash at the Tour De France a few weeks ago," he said.
"It was a very technical course and it was really hard. In the end, I still felt a lot of pain in my back and in my hip. I think I really needed more time to recover for this race."
Italy also won the women's road race last week with Marta Bastianelli outsprinting defending champion Marianne Vos (Netherlands) and Lisa Brennauer (Germany).