The 35-year-old won ahead of Sarah Roy (Mitchelton-Scott) and Charlotte Becker (Hitec Products) after a successful breakaway during the 100.3km, 17 laps, street circuit race.
"I cannot believe it!" Bronzini said. "It was my dream and it's come true. It was only possible through the amazing work of my teammate Rossella (Ratto) who worked for me.
"The perfect move for me was the break. I design a day like today to finish my career, it was amazing. Icing on the cake!"
Ellen Van Dijk (Sunweb) won the overall classification after her team dominated Saturday's opening teams time trial.
"Our original goal was to go for the intermediates with Coryn but when the big break went, we had to adjust the plan," Van Dijk said. "
"We still wanted to have the breakaway back before the final to sprint with Coryn, so the girls tried to chase in the bunch but there wasn't enough help, so me and Lucinda stayed away in the break until the finish.
"We can be really happy with 1-2 in the GC and the other girls close behind, especially after our TTT win yesterday. It's been a good weekend with the team with lots of good points to take forward to the worlds.
"I'm really happy to win a WorldTour race for the team and to finish my time here on a high note."
The peloton controlled the stage until Coryn Rivera Sunweb) won the first intermediate to move into the virtual overall lead off the back of Sunweb's time trial win on Stage 1. The American sprinter was also second on the second intermediate sprint but ultimately couldn't hold that position.
Rozanne Slik (FDJ Nouvelle-Aquitaine Futuroscope) took the third intermediate sprint from the breakaway but was caught along with her breakaway companions after an attack from Elisa Longo Borghini (Wiggle High5) with eight laps to go. A group of 19 attackers featuring Van Dijk then jumped ahead to a 50 second lead with 27km to go.
The gap stabilised and Malgorzata Jasinska (Movistar) and Coralie Demay (FDJ Nouvelle-Aquitaine Futuroscope) accelerated with 16km to go. It then increased to one minute before Ale Cipollini reacted at the front of the bunch, reducing it to a handful of seconds but could not reach Bronzini who edged out Roy at the line.