Three days ago, Cook challenged his home state of Alabama to better ensure the rights of the lesbian, gay, bisexual and transgender community.
Alabama is among the states that do not recognize same-sex marriage, and it also doesn't offer legal protections on the basis of sexual orientation or gender identity. Cook is a native of Robertsdale, Alabama, and attended Auburn University.
The announcement is a "huge deal," said Richard Metheny of executive search firm Witt/Kieffer.
"This really sets the stage for 'It's OK,' " he said. "Anything CEOs do is very magnified, very complicated, and it affects a lot of people. ... There's no taking away that he has become a role model and will have a positive influence on lots of people that would like to be comfortable being out in the world of business."
"I consider being gay among the greatest gifts God has given me," Cook wrote in the essay Thursday.
The executive said that "being gay has given me a deeper understanding of what it means to be in the minority and provided a window into the challenges that people in other minority groups deal with every day."
Cook said he's been lucky to work for a company that "loves creativity and innovation and knows it can only flourish when you embrace people's differences."
Cook, 53, succeeded Apple founder Steve Jobs as CEO of Apple in 2011.
Apple has been an outspoken champion for diversity since Cook succeeded Jobs as CEO. The company has trumpeted the phrase, "Inclusion inspires innovation," as a rallying cry. Cook has reinforced that message on his Twitter account with periodic posts supporting gay rights in the workplace.
Cook's public declaration that he is gay comes a little more than two months after Robert Hanson — the former CEO of American Eagle Outfitters Inc. -- wrote a piece for Time in which he talked about being an openly gay man for as long as he's been in business and running companies.
Hanson is currently the CEO of luxury jewelry brand John Hardy.
There are no other publicly gay CEOs of major companies. United Therapeutics Corp. CEO Martine Rothblatt, who was born male and is now female, has been open about her transgender status.