Police said they arrived at 42nd Ave. S.W. and S.W. Graham Street after receiving a report that the woman was struck by the BB at 8:02 p.m.
The woman, her husband, and their child went out for a walk in their neighborhood when they heard what they thought was someone firing the BB gun at tin cans, Det, Jeff Kappel said.
Soon after, the woman was struck on the butt by a BB, police said. Her husband believed he saw someone fire a BB-type rifle from the window of a nearby apartment complex and pointed that out to officers who checked it out.
Officers found a man in a second-floor apartment and determined he was the one shooting the BBs from above. He was aiming down at some aluminum cans shooting the BBs from an Airsoft rifle.
One of the plastic rounds from the rifle bounced off a can and hit the woman in the butt.
Officers arrested the 20-year-old man and booked him into the King County Jail for reckless endangerment, which is a gross misdemeanor.
Police took several of the man's Airsoft guns and a bag of plastic BBs for evidence.
The woman was not injured and turned down medical treatment.