The incident occurred at the Josephine Sunset Home, located in the 9900 block of 272nd Place NW, just after 3 p.m.
Snohomish County sheriff's spokeswoman Shari Ireton said a resident who had gotten into an argument with the director earlier in the day returned with a gun and opened fire.
The 59-year-old victim was airlifted to Harborview Medical Center in critical but stable condition.
Snohomish County Sheriff's Office detectives arrested an 86-year old man thought to be responsible for the shooting. Detectives say he will booked into the Snohomish County Jail for 1st degree assault.
"Everyone's safe. This was an isolated incident," said Terry Robertson, CEO of the facility.
Robertson said the facility does not allow firearms, but did not elaborate on how the suspected gunman gained access to the weapon.
"That could be part of the investigation. I can't really speak to that but our policy is no handguns, no rifles, no any firearm of any kind," Robertson said.
The state Department of Social and Health Services planned to launch its own investigation. A spokesman for the department said the shooting happened in the assisted living side of the building, where the suspect lived.
About 55 people live on that side of the building, and 150 others live in the nursing home section. There's also a child center, but no one else was injured during the shooting.