Penguins general manager Ray Shero announced Friday that Letang had a stroke last week.
"Kris had one brief episode of dizziness and nausea last week," Shero said. "We held him out of the Los Angeles game Thursday night, and when he continued to feel ill, tests conducted in Phoenix on Saturday gave us the first indication of his condition.
Shero said doctors found a small hole in the wall of Letang's heart during further medical examinations, and that they are seeking to assess if it led to the stroke.
Letang will be re-evaluated after a six-week treatment period. He was given clearance to go on vacation with his family during the NHL's Olympic break.
"Further testing then was conducted when he returned to Pittsburgh, and he continued to undergo a battery of tests here this week."
Letang will be treated with blood thinners and the stroke is not expected to threaten his career.
"I hope that by making my condition public at this time, I can help other people by encouraging them to seek medical help if they experience some of the symptoms associated with a stroke -- regardless of their age or general health," the 26-year-old Letang said. "It obviously was a shock to get the news, but I'm optimistic that I can overcome this and get back on the ice."
Letang's most recent game was Jan. 27 against Buffalo.
After being a Norris Trophy finalist last season, he has 10 goals -- matching a career high -- and 18 assists in 34 games this season for the Eastern Conference-leading Penguins.
Letang has 54 goals and 227 points in 419 games since making his NHL debut with the Penguins during the 2006-07 season.
The Montreal native signed a $58 million, eight-year contract extension last summer.
"The most important thing right now, of course, is Kris' health," Shero said. "We're not thinking about hockey right now. We want to make sure he gets the best possible care and gets better. After six weeks of treatment, doctors will re-evaluate Kris."
Boston Bruins forward Jarome Iginla said he was shocked after hearing the news of Letang's stroke. The Bruins practiced Friday afternoon at Ristuccia Arena, and afterward Iginla was in the players' lounge when he saw the report on television.
"It's a scary thing," Iginla said. "He's a young guy and he takes care of himself and he's in good shape. I don't know anything more than what I saw on TV, but yeah, it's obviously more serious than a hockey injury. I'm definitely shocked and wish him well. It was definitely a surprise and a shock to see."
The Associated Press and ESPNBoston reporter Joe McDonald contributed to this report.