The pop star shared the news on Instagram on Sunday.
"Look at that. Do you like it?" Sam asks in the video.
"Yes!" Britney yells back excitedly.
"I can’t f**king believe it 💍💍💍💍💍💍❣️!!!!!!" Britney captioned the post, as fans shared their well wishes for the couple in the comments.
Sam also shared the news on his Instagram, alongside a close up of the ring on Britney's finger. "🤴👸 @britneyspears," he wrote.
He later tool to his Instagram Story to thank everyone for their well wishes. "My phone exploded today! Thank you for the overwhelming love and congratulations," he posted. "We both love [every] single one of you!"
In a statement to ET, Sam's manager Brandon Cohen said Sam and Britney are "deeply touched by the support, dedication and love expressed to them."
"Roman Malayev from NYC is the ring designer," Cohen added. "He couldn’t be more happier to be involved and make this a one of a kind ring."
Britney's stunning sparkler is a 4 carat diamond ring designed by Malayev at Forever Diamonds NY, ET has learned. Sam spent months finding the perfect designer for Britney's custom ring, which has a sentimental engraving -- the word "Lioness" (Sam's nickname for Britney) -- on the inside of the band.
"Every designer I spoke with about the ring was amazing with great ideas, but I just clicked with Roman -- we really connected on the design and he was as excited as I was to make it special," Sam said in a statement. "That’s why I chose him."
Britney and Sam began dating after they met in October 2016 on the set of the singer's "Slumber Party" music video. The 27-year-old Iranian-American actor played her love interest in the visual, before becoming her real life beau. A source told ET earlier this month that after nearly five years together, they were ready to get engaged.
"Britney and Sam are an end game couple. But for them, it’s all about timing," ET's source said.. "All of their dreams for their relationship have been on pause due to the conservatorship. They haven’t been able to progress like a normal, adult couple. It’s like dating in high school. That’s been their experience for the last two years or so."
"Things are finally looking up and it’s starting to become very real for them that soon they'll be able to take back the reins and have full control as a couple," the source continued. "They are ready to embrace that next chapter, and love each other without limits or without feeling like they are on a leash. Naturally, an engagement is the next step."
The source added, "It’s never been a question of, 'Will it happen?' It’s always been about when."
"They're finally getting closer to the 'when' thanks to Britney’s conservatorship suit yielding in her favor. It’s all very real for Sam especially, and engagement is on his mind now more than ever. He and Britney want that badly," the source noted, adding, "They want the whole family thing too. It’s just a matter of timing."
Sam has been Britney's biggest ally amid her recent conservatorship hearings, showing his support for his lady on social media. Britney recently praised Sam for being with her "through both the hardest and the best years of my life."
The pair's relationship also played a significant role in Britney's explosive courtroom testimony earlier this year, when Britney claimed that, under her conservatorship, she is unable to get married or have another baby.
"Britney and Sam basically live their life as a couple under curfew. She wants to be free of all the restrictions and she is ready to fight for her life because that is what this is," a source previously told ET. "Sam feels so proud of Britney for finally having this moment. It's her liberation and it's a long time coming."
In a 2019 interview with ET, Sam expressed his hopes of one day getting married to Britney.
"Absolutely," he responded when asked whether he saw marriage in his future with the singer. "This is something that every couple should do. That's the whole point of a relationship -- we are a family.”
"Trust me, if we do ever get married, everybody’s going to know," he added. "Actually, you might be the first one to know -- you never know!"
See more on Britney and Sam in the video below.