As OK! previously reported, the Flip or Flop star announced the engagement on Instagram on Monday, September 20.
The blonde beauty announced the good news with a series of photos with Hall. She put her arm on her man's shoulder in the last photo to show off her new rock. "❤️♾🔒🗝 💍," she captioned the post.
Her costar and ex-husband seemed to have learned about it on social media, telling PEOPLE, "We just saw, but congratulations!"
"Wow, awesome! We just want nothing but happiness for [Christina and Josh] and nothing but happiness for the kids," Young told the publication. "We're looking forward to the next chapter of everyone's lives."
Haack and El Moussa tied the knot in 2009 and called it quits in 2016. They share children Taylor, 10, and Brayden, 6.
Haack and Hall's romance was confirmed in July, one month after her divorce from her second husband, Ant Anstead, was finalized. The former flames share son Hudson, 2. The Wheeler Dealers host is now dating actress Renée Zellweger.
Anstead previously told PEOPLE that he is a big fan of "modern families" and still considers Taylor and Brayden to be his step-children.
"I think it's amazing how generations evolve to fit within the current times, and right now, we live in a time where blended families are common and ultimately you have to have relationships that are healthy with all this extended family. And by default, I've inherited an extended family [in Tarek and Heather]," he said at the time.
"To treat them in any other way would be dishonorable," the 42-year-old added. "I'm really proud of how we all kind of manage that modern blend."
Meanwhile, El Moussa and Haack reportedly butted heads while filming Selling Sunset earlier this year. In July, he reportedly lashed out at the blonde beauty while filming and called her a "washed up loser."
"As you can imagine, it can be stressful working with an ex and we did have an incident where some choice words were said on both sides and I’m sure we both wish the whole thing never happened," the HGTV star said on E!'s Daily Pop.
He vowed to never let their arguments escalate like that again. "I mean, I certainly do, but I mean, honestly, since we got into that little disagreement a few months back, I’ve just decided that moving forward, never again," he explained.