Many of the couple's famous friends and family members couldn't contain their excitement over the big news.
The rocker popped the question on the beach in Montecito, California, and the reality star shared a photo of the romantic proposal to Instagram, where it was met with a lot of love.
Kourtney captioned the photo -- which revealed the seaside proposal inside a heart-shaped ring of red roses and candles -- simply, "Forever @travisbarker."
Quickly, there was an outpouring of support in the comments. TikTok star Addison Rae - -who shared a photo of the proposal to her Instagram story, commented, "IM SCREAMING."
Meanwhile, model Winnie Harlow wrote, "Ahhhhh 😍congratulations," while Kourtney's sister, Kim Kardashian, posted two comments -- one featuring heart emojis and one featuring engagement ring emojis.
Kim later took to Twitter to share a video of the love birds sharing a kiss at their engagement dinner, and this also gave fans the first good look at Kourtney's sizable sparkler.
"KRAVIS FOREVER," Kim captioned the post, along with a diamond ring emoji.
Meanwhile, Travis' two children, 17-year-old Landon and 15-year-old Alabama -- whom he shares with his ex-wife, Shanna Moakler -- took to their Instagram stories to share photos from the engagement celebration, and messages of love and support.
"Congratulations @kourtneykardash and @travisbarker I'm so happy for you guys love you guys so much!" Landon wrote over a video of the pair exchanging a kiss at the table.
Alabama, meanwhile, shared a photo of her dad giving Kourtney a sweet peck on the cheek.
"So happy for you guys. I love both!" she wrote.
A source told ET on Sunday that the pair couldn't be happier about taking this next step in their relationship.
"Kourtney and Travis are over the moon. Getting engaged was always their natural next step and they are so thrilled," the source said. "The proposal was gorgeous and romantic, and everything Kourtney wanted."
"Their families are beyond happy and excited for them too. Their kids are very much looking forward to being an official big happy family," the source added. "Kourtney and Travis really feel like they've found their perfect partner and match. They can't wait for this next step in their journey as a couple and to spend the rest of their lives together."
Kourtney, 42, and Travis, 45, have been longtime friends and neighbors, and started dating in January. The two quickly got serious, taking family trips with their respective kids. Kourtney shares three children -- 11-year-old Mason, 8-year-old Penelope and 6-year-old Reign -- with her ex, Scott Disick.
A source told ET in May that the couple had discussed a possible engagement and marriage, and that the Kardashian family approves of Travis.
For more on the cute couple, check out the video below.