"I can confirm Alex has made the personal decision to retire," TransCanada spokesman James Millar said. "He will be (at) the company until the end of May."

Pourbaix was previously president of development for TransCanada, responsible for overseeing growth projects including the long-delayed Keystone XL oil pipeline, which finally received a U.S. presidential permit last week.

TransCanada declined to comment on whether the Keystone XL approval had anything to do with the timing of the announcement from Pourbaix, who was considered by some industry players to be a potential successor to TransCanada's chief executive officer Russ Girling.

"I always thought he was one of the guys that would be in the running, but there are a number of very, very capable people at TransCanada," said AltaCorp Capital analyst Dirk Lever, adding he was surprised by the move.

(Reporting by Nia Williams; Editing by Cynthia Osterman and Bill Rigby)