Tori Spelling and Dean McDermott are calling it quits on their nearly two-decade marriage.

Dean announced their breakup Friday night on social media, saying ... "It’s with great sadness and a very very heavy heart that after 18 years together and 5 amazing children, that @torispelling and I have decided to go our separate ways, and start a new journey of our own."

The Canadian actor added ... "We will continue to work together as loving parents and guide and love our children through this difficult time."

It doesn't appear either party has filed anything in court yet.

So far, Tori has been mum about the split ... but, to Dean's point about co-parenting, the now estranged couple was photographed celebrating their daughter Stella Doreen's 15th birthday earlier this week at the Beverly Hilton.

They also share 4 other kids ... Hattie Margaret, 11; and sons Liam Aaron, 16, Finn Davey, 10, and Beau Dean, 6,

As you might recall, the "Beverly Hills, 90210" alum tied the knot with Dean back in 2006, and they renewed their vows 4 years later ... but their relationship has been rocky throughout.

In fact, Dean admitted to cheating on Tori, explaining that he was "out of control" and "sex was an escape." The couple even talked openly about Dean's infidelity on their own reality show, "True Tori."