The shooter was also killed, said James Rhodes of the Clackamas County Sheriff's Office.
Panicked customers ran for cover when shots rang out at the Clackamas Town Center mall.
"Multiple victims" were shot, said Happy Valley, Oregon, Public Safety Director Steve Campbell. There were 10 ambulances on scene after the shooting, Randy Lauer of American Medical Response said.
Witnesses told CNN affiliates that many people ran for cover or huddled behind counters at the two-story mall, which is located about 11 miles southeast of downtown Portland.
A woman told CNN affiliate KOIN that she saw a man wearing a hockey mask jogging through Macy's and wielding an assault rifle.
"Everyone ran toward the exits at that point to get out," she said.
Inside Sears, some customers cried as word spread of the shooter going store to store, Christina Fisher told KOIN.
"We were told to stand in a group by the top of the escalators and stay away from the windows out of the aisle. ... We stood there for probably a good 20 minutes," she said. "All of the sudden, somebody came through with a radio, yelling 'get down!'"
A group of customers inside Sears watched television news reports about the shooting inside the store's entertainment center, witness Tylor Pedersen told CNN affiliate KGW.
Pedersen said he heard about the shooting when people ran into the store, saying they had heard shots fired in the middle of the mall.
"I didn't think it was real at first until I saw the reactions on their faces," he said. "They were serious."
Nicole Sutton said she heard gunshots echoing and saw people huddled in the store where she works at the mall.
"It's the scariest thing I've ever experienced," she told KOIN.
Oregon Gov. John Kitzhaber praised first responders for reacting quickly to the shooting.
"I have directed State Police to make any and all necessary resources available to local law enforcement," he said in a statement.
Authorities closed entrances and exits into the mall parking lot Tuesday evening, said Lt. Gregg Hastings of the Oregon State Police.
CNN's Cristy Lenz, Tom Watkins, John Fricke, AnneClaire Stapleton and Joe Sutton contributed to this report.