According to the Kennewick Fire Department, it happened around 10 a.m. at an apartment building in the 700 block of North Arthur Street.
Authorities say the boy landed on the concrete sidewalk below the window and received a critical head injury. He was transported to Sacred Heart Medical Center in Spokane.
KFD says it appears the boy had climbed up onto a couch to watch a passing train when he fell through the window. The window had a screen, but it was wide open.
The boy's mother was in the room, but authorities say she had her back turned to him when he fell.
Officials say this is at least the third case in the last three years of a child falling from a second-floor window in the Tri-Cities area. They advise parents not to place objects on which children can climb close to windows and to keep upper-story windows only partially open.