In a since-deleted Instagram post, the 33-year-old showed a picture of the new ink, which read "Yes sir, Mr. El Moussa" in cursive and was captured by various outlets before she took it down.
"Happy Valentine's Day Mr. El Moussa ❤️💍 @therealtarekelmoussa," Young captioned the post. "I love you handsome! Best Friends Forever."
"Forever and ever and ever❤️❤️❤️❤️❤️❤️," the Flip of Flop star replied to his fiancée in the comments. "Love you so much," Young shot back.
"This is a vibe," Lindsie Chrisley Campbell commented, to which Young reportedly replied back with a heart emoji.
The pic also gave her followers a good look at her 8-carat engagement ring, which recently had an upgrade when the couple designed their ideal band together.
The Selling Sunset star explained in since-removed Instagram Stories that the tattoo was a surprise for the 39-year-old.
"Obviously you guys all saw that I got a tattoo, so I'm gonna post a cute little video that I did right before I got that tattoo," she said in a video on Tuesday, February 9.
"So this man over here, his name is Sean Arnold, and he's doing a tattoo for me," the blonde beauty told her 1.6 million followers.
"It's very meaningful because I'm doing it for my fiancé," Young explained. "It's something that's just for us, so you might not see it. But this guy right here is going to hook me up."
Although she was "really, really nervous" to get the tattoo, the energy in the tattoo studio was "really calming." "[Arnold] lit some Palo Santo," she added. "He has crystals everywhere."
El Moussa popped the question on July 25, 2020, as the couple celebrated their first anniversary on a boat trip to Catalina Island, which is off the coast of Southern California but they are tight-lipped about when they plan on tying the knot. Young gushed on social media that she already found a dress with help from Chrishell Stause.
El Moussa and Young were first linked in summer 2019 after El Moussa split from Christina Anstead.
The ceremony will be "late this summer, complete with a lavish reception, gourmet menu and plans to sail away on their very own love boat!" a source previously told OK!.
El Moussa was even said to tell Young that she is free "to spend what she wants — money is of no concern — and that he'll even foot the cost for the bridesmaids' dresses and pre-wedding parties. She makes him feel incredible, and that's going to be reflected in the wedding they're planning."