"In the ultimate plot twist...HE SAID YES!" Becca, 32, captioned a slideshow of photos featuring Thomas' new gold ring, the couple popping champagne together, and the lovebirds' two dogs.
Becca didn't specify when exactly she and Thomas got engaged, as she continued in her post, "We've been keeping this secret just between us and close family & friends for a while but we're so excited to shout it from the rooftops!"
"I finally found my chauffeur for life and the one who makes my heart smile every single day," she added. "I can't wait to do it all with you Tommy, thanks for making me the happiest gal alive. I love you to the moon & back."
Meanwhile, Thomas, 29, posted similar photos on his own Instagram account, writing, "The ultimate UNO reverse card / power move. You keeping me on my toes for a lifetime has a great ring to it."
"Cheers to forever Boops #Fiance," Thomas concluded.
After starring on The Bachelorette's fourteenth season in 2018 and working through two broken -- and very public -- engagements to former The Bachelor star Arie Luyendyk Jr. and The Bachelorette 14 winner Garrett Yrigoyen, Becca looked for love on Bachelor in Paradise's seventh season last summer and met Thomas.
On Bachelor in Paradise's Season 7 finale, Becca was shown breaking up with Thomas and crushing his heart before overnight Fantasy Suite dates because, despite having "an incredible start" to a relationship, Becca worried their romance was "too good to be true."
Becca didn't want to leave The Bachelor franchise for the third time with a man unless she knew everything about him, including how he'd face adversity in the real world.
On Becca's way out of Paradise, a teary-eyed Thomas admitted he was "falling in love" with her and had so much faith in what they had and what their future could be like together, which made Becca question her decision and feel regretful.
Bachelor in Paradise's finale concluded in October 2021 with an update on Becca and Thomas' relationship that revealed the pair had reunited after filming ended and are now dating, "happy and in love."
"When we were dating on the beach for that short amount of time, everything was smooth sailing. It was perfect... But for me, I needed more of the real life. I think to go through that breakup in that very short courtship ultimately brought us back together."
Becca and Thomas then began spending time together in San Diego, CA, and they both found the clarity and answers they needed.
"I'm very aware [Becca] wants an elongated cushion [cut diamond]. I'm very aware she wants it to be very large," Thomas revealed.
"I don't have a full plan yet, but I know it's going to be small and intimate... The last thing I want for Becca is to have another grand spectacle."
He added, "Am I aware if I have one in a safety lockbox in my bedroom? I don't know, time will tell. We'll have to find that one out later on this year."
Back in early November 2021, Thomas told Hollywood Life that he was pretty sold on marrying Becca because they were spending a lot of time together and had "the most healthy, normal relationship" at the time.
During an October episode of the "Talking It Out with Bachelor Nation" podcast hosted by The Bachelorette alums Mike Johnson and Bryan Abasolo, Becca revealed how Thomas has sent her "multiple text messages" asking about engagement rings.
"It's something we've talked about," Thomas disclosed during the joint interview. "I mean, I don't want to spend my life with anybody else... I wouldn't want to wake up next to anybody else."
Before falling in love with Thomas, Becca had her heart broken by Arie on The Bachelor's 22nd season in 2018, when he decided to end their engagement because he was still in love with his runner-up, Lauren Burnham, who is now his wife and the mother of his three children.
Becca then accepted a marriage proposal from Garrett during the finale of her The Bachelorette season but they split up in mid-2020.
But the third time appears to be the charm considering Becca has been thinking for nearly a year now that she and Thomas are meant to be.
"I'm gonna probably hate myself for actually admitting this to anyone, but I called my mom -- and even though I was engaged twice in the past -- I was like, 'Mom, I think I want to marry this guy,'" Becca previously said on the "Talking It Out" podcast.
"It's the first time I've ever actually said that out loud."
Since the pair began dating in the real world, Becca has defended Thomas and stood up for their relationship on more than one occasion considering he had been portrayed as a villain on Katie Thurston's season of The Bachelorette.