Olympic champion, Ecuadorian Richard Carapaz (Ineos), retained the leader's pink jersey two days before the final time trial on Sunday in Verona, with the same advantage of 3 seconds over Australian Jai Hindley.
Bouwman beat his four breakaway companions in a bumpy final due to a "straight" for two riders (Vendrame, Valter) in the last corner.
The Dutchman, who secured his best climber's jersey, beat the Swiss Mauro Schmid and the Italian Alessandro Tonelli.
Hungarian Attila Valter took 4th place ahead of Italian Andrea Vendrame at the end of this mid-mountain stage which made an incursion into Slovenia.
The breakaway, strong of 12 riders, approached the main climb of the day, the Kolovrat (10.3 km at 9.2%), with more than nine minutes ahead of the peloton.
The four survivors (Bouwman, Schmid, Tonelli, Valter) only lost a little over a minute on this climb near Caporetto, the site of a historic defeat for the Italians in the First World War.
The advantage then grew when Australian Jai Hindley's Bora team dropped out of the lead, but narrowed on the 7 kilometer long final climb.
Carapaz started several times without managing to get Hindley.
The Spaniard Mikel Landa, third in the standings, also tried without success.
Bouwman, 28, won for the second time in this Giro after his success at Potenza (south), in the 7th stage.
Saturday, the 20th stage, the last mountain, includes three great climbs of the Dolomites, three historic passes: the San Pellegrino, the Pordoi for the highest point reached by this edition at 2,239 meters above sea level, the Fedaia in conclusion.
This last ascent (14 km at 7.6%) leads to the heart of the Marmolada massif in the Dolomites by a spectacular climb, very steep in its last 5400 meters (11.2%).
© 2022 AFP