It would be the third drowning in the river this month and the second since Friday.
Sheriff's Lt. Chris Spallino said the man disappeared under the water around 9 p.m. off Dubrava Way near Vacation Beach in the Guerneville area.
On Friday 21-year-old Jose Antonio Molina of Santa Rosa drowned in the river north of Railroad Bridge near Memorial Beach in Healdsburg.
Healdsburg police said Molina was walking along a shallow area and fell into a deep area. Molina did not know how to swim, police said. His body was pulled from the water around 2:11 p.m., police said.
Oakland resident Carlos Manzo, 39, drowned in the river near Monte Rio Beach on June 13 when he tried to rescue his 10-year-old daughter. The girl was rescued by a woman on the beach.