The finish was so close that Cimolai did not celebrate victory until minutes after crossing the finish line of the 178.9 km race, which started and ended in Calella and consisted of six major climbs.
Kristian Sbaragli of Dimension Data came third.
"I did not know where I was,” Cimolai said. "Milan-San Remo was very long and tiring but I felt really good in the final today.
“I want to thank my whole team [which] was fantastic for me. This is my second victory here, which was the goal. Now I will work for my teammates.”
The original break of four riders swelled to seven but the group was unable to make much headway, with the sprint teams keeping them on a tight leash.
Cyril Gautier (AG2R La Mondiale) and Peter Kennaugh (Sky) tried to sneak away late in the game but lost out to a fast moving peloton as it prepared for the closest of sprints.
Chris Froome (Sky) made his European racing debut of the season and is one of the favourites to win the general classification along with Alberto Contador (Trek-Segafredo) and Alejandro Valverde (Movistar).
Stage 2 is a 41.3km time trial in Banyoles where it is expected the general classification riders like Froome will establish their positions for the rest of the Tour.