Saretsky, who has headed WestJet since 2010, will retire.
"Gregg Saretsky, president and CEO, has advised the time has come for him to retire from the company. Having found his successor, he has agreed with the company that his retirement will be effective immediately," the airline says.
"With plans well underway for the launch of Swoop and the introduction of the 787-9 Dreamliners on the horizon… I'm confident WestJet will continue to grow to the next chapter and beyond," says Saretsky.
Saretsky joined WestJet in June 2009 as vice-president of WestJet vacations. He became the company's executive vice-president of operations in October 2009, then became chief executive in March 2010.
A newcomer to WestJet, Sims joined the airline as executive vice-president of commercial in May 2017, taking a position previously held by Bob Cummings, who transitioned to head WestJet's new ultra-low-cost subsidiary Swoop.
Sims previously had been chief executive of Airways New Zealand, that country's air service provider. He also worked at Tui, Thomas Cook, Virgin Group and Air New Zealand, leading the latter's international widebody business, WestJet notes.
"I am honoured by the opportunity to assume the role of president and CEO, and believe strongly in WestJet's next global chapter and the growth potential ahead," says Sims. "WestJet has a strong challenger's spirit."
The change comes at a pivotal moment for WestJet, which is executing a business transformation executives say will help it compete at all spectrums of the air travel market.
That transformation includes the launch of Swoop, aimed at attracting price-sensitive travellers. Swoop is scheduled to operate its first flight on 20 June.
Meanwhile, the company is going upscale and international. WestJet is preparing to take delivery of 10 Boeing 787-9s, an aircraft that will help the largely-provincial carrier expand overseas. WestJet is also building business class lounges and has discussed adding business class to its 737s.
(Jon Hemmerdinger - FlightGlobal News)