'RHOC' Star Alexis Bellino Engaged to Shannon Beador's Ex John Janssen

"The Real Housewives of Orange County" star Alexis Bellino should expect tension with Shannon Beador at the reunion ... 'cause she just agreed to marry her costar's ex.

The Bravo personality announced her happy news on Instagram late Wednesday evening, confirming her boyfriend, John Janssen, proposed.

Per Alexis, John popped the question while they were celebrating their 9-month dating anniversary ... which we didn't know was a thing, TBH.

Nonetheless, AB couldn't be more thrilled to spend her life with the businessman.

She wrote ... "Every moment together has been a gift.  am so in love with you and the beautiful life we’re building together with our kids. And the answer is 100000000% yes."

Alexis and John first got together in November 2023 ... not long after JJ parted ways from on-again, off-again girlfriend Shannon Beador following her DUI arrest. Alexis was also newly single, as she had called off her engagement to Andy Bohn earlier in the fall.

Alexis' decision to date John has been a hot-button topic on season 18 of 'RHOC' ... with the women regularly butting heads over their shared lover -- and the $75K Shannon is said to owe JJ for a past facelift.