The pilot, identified as 63-year-old Henry Skillern of Humble, Texas, suffered a sudden heart attack during a flight from Houston to Seattle. He died after he was taken to Saint Alphonsus Regional Medical Center for treatment.
The Boeing 737, carrying 161 passengers and a crew of six, made a safe emergency landing in Boise around 8 p.m. PT. Another pilot flew the original plane and passengers to Sea-Tac Airport, according to United spokeswoman Christen David.
Skillern had been a pilot with United Airlines for 26 years, David said.
The Associated Press contributed to this report.