Union Hotel and Union Hall Site is historical site of buildings built in 1850, in Sonoma, California in Sonoma County, California. The Union Hotel and Union Hall Site is a California Historical Landmark No. 627 listed on January 13, 1958. The original Union Hotel was a one-story adobe built by three veterans of the Mexican–American War. Next to the Union Hotel was built the Union Hall. The two buildings were lost in 1866 fire. The Union Hotel was rebuilt into a two-story stone hotel. The hotel was on the second floor and a hall was on the first floor. The Union Hotel and Union Hall were on the south side of the Sonoma Plaza.