The Team DSM rider held off Jonathan Milan (Bahrain Victorious) in a mass sprint to prevail by a few centimetres, with Michael Matthews (Jayco-AlUla) taking third place.
"In the last metres, I was really digging so deep. I was really on the limit and I saw Johnny coming. I couldn't throw my bike because I was really on the limit but it's nice to get a few centimetres in front and get the win," Dainese said.
"In the last five days, I was quite sick with stomach issues. Today was the first day I was feeling okay, like 80%, and to win after such a struggle in the last five days is insane."
Briton Mark Cavendish, who announced on Monday that he will end his 17-year career as a professional cyclist at the conclusion of the season, also appeared to be in contention for the stage win, but faded in the final kilometre.
A four-man breakaway comprising of Thomas Champion (Cofidis), Charlie Quarterman (Corratec-Selle Italia), Diego Sevilla (Eolo-Kometa) and Senne Leysenc (Alpecin-Deceuninck) built an advantage of over two minutes.
Leysen attacked with 22km to go and put in a valiant performance as he battled a challenging headwind, but he never stood a chance as Movistar and Groupama–FDJ took control of the surging peloton, which swallowed up the Belgian rider with 5km left.
Ineos Grenadiers rider Geraint Thomas continues to lead the overall standings, 18 seconds ahead of Joao Almeida (UAE Team Emirates) with Primoz Roglic (Jumbo-Visma) in third place.
"It was a crazy bunch finish. Luckily it didn't rain that much today," said Thomas, who turns 37 on Thursday.
"We are all safe and ready for the next three stages. Wearing the Maglia Rosa on my birthday will be definitely nice, I hope I can keep it until Rome."
Thursday's stage 18 takes place on a mountainous 161-km route from Oderzo to Val Di Zoldo, with two category one ascents.
