Around 3:10 p.m. the man calmly entered the bank at 424 Farmers Lane and presented a teller with a note demanding money. He received an undisclosed amount of cash then left the bank and walked away, according to police.
He was wearing a baseball cap, a hooded sweatshirt and dark jeans. He was also wearing what appeared to be prescription glasses, police said.
The Sonoma County Alliance, through its "Take Back Our Community" program, is offering a $2,500 reward for information leading to the robber's arrest.
Anyone with information on the crime or who can identify the suspect is encouraged to contact the Santa Rosa Police Department's Violent Crimes Team at (707) 543-3590.