The Penguins made the announcement on their website, adding his condition is treatable with blood thinners and at this point isn't believed to be career threatening.
Letang has been cleared to go on vacation with his family during the NHL's Olympic break.
Further testing also showed Letang, 26, has had a very small hole in the wall of his heart since birth.
This defect is present in all individuals before birth but usually seals shut in most cases and it's possible the condition led to the stroke.
"Kris had one brief episode of dizziness and nausea last week," Penguins GM Ray 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.
"Further testing then was conducted when he returned to Pittsburgh, and he continued to undergo a battery of tests here this week."
The six-foot, 201-pound Letang was a Norris Trophy finalist last year as the NHL's top defenceman and has 10 goals and 18 points in 34 games this season.
Letang has played 419 career games over seven seasons, registering 227 points, including 54 goals.
"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," 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."
After six weeks of treatment, Letang will be re-evaluated.