Doohan, 84, was diagnosed with Alzheimer's "within the last couple months," agent Steven Stevens told The Associated Press.
He said Doohan is in the beginning stages of the disease, a progressive neurological disorder that afflicted former President Ronald Reagan, who died June 5.
Doohan also has suffered for some time with Parkinson's disease, diabetes and fibrosis due to chemical exposure during World War II and the D-Day invasion in France, Stevens said.
He lives in Redmond, a suburb northeast of Seattle.
Stevens said he last saw Doohan in January in Los Angeles when the actor made a cameo appearance in the upcoming horror film "Skinwalker: Curse of the Shaman."
"He didn't have any energy and he seemed very frail. But as soon as they yelled 'Action!' he was the same old feisty Scotty," said Stevens, who has represented Doohan for 28 years.
Doohan's career spans more than 50 years, but he's best known for his role as the USS Enterprise's affable chief engineer, Lt. Cmdr. Montgomery "Scotty" Scott, in the original 1966-69 "Star Trek" TV series.
He's also appeared in several "Star Trek" movies.
Doohan has lived in Redmond for almost a dozen years with his wife, Wende. They have a 4-year-old daughter and two older sons, and Doohan has four children from a previous marriage, Stevens said.
Doohan is scheduled to receive a star on the Hollywood Walk of Fame Aug. 31.
He plans to attend a three-day "Star Trek" farewell convention, Aug. 28-30, in Hollywood, Stevens said. All surviving members of the original Enterprise crew are scheduled to attend, including William Shatner, who played Capt. James T. Kirk, and Leonard Nimoy, who was Mr. Spock.
By Melanthia Mitchell