The former Olympian stirred up controversy in 2015 when she told Ellen DeGeneres that she believed marriage should be between a man and a woman, but since transitioning, Jenner, 67, has changed her position.
"I am 100 percent behind gay marriage. Let's clear that up right now," the I Am Cait alum told Sawyer, 71, in a clip for her 20/20 interview airing on Friday, April 21.
She admitted it was an evolving process for her, but is now "all for it."
The conservative reality TV star — whose new memoir, The Secrets of My Life, is out on April 25 — also talked about her support for President Donald Trump in another clip. The longtime Republican admitted she has had a perspective change when it comes to politics too.
"Yes, I did vote for Trump, but here's the deal breaker with the Republican party," Jenner said. "And the deal breaker is, you mess with my community, you do the wrong thing with our community, you don't give us equality and a fair shot, I'm coming after you."
"When it comes to all equality issues, for the entire LGBT community, what we need is federal guidance," she continued. "Just like the previous administration said that it was OK to serve as a trans person in the military. We have frontline people, OK? I'm talking Marines, trans guys, on the frontline, fighting for our country. I'm trying to get, especially the Republican party, to make a change."
Sawyer asked the Keeping Up With the Kardashians star if she has spoken to Trump secretly or privately about trans issues.
"When I was at the inauguration I did say hi at a cocktail party, and he wanted me to play golf with him," she revealed. "At the time I thought it was a pretty good idea but since Title IX, it's not a good idea and so I won't be playing golf with him."
Caitlyn Jenner: The Secrets of My Life airs on ABC, Friday, April 21, at 10 p.m. ET.
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