Sources with direct knowledge tell us, Tristan's mom, Andrea, suffered a heart attack at home Thursday ... she was rushed to a local hospital where medical experts tried to resuscitate her, but she didn't make it.

Our sources say Tristan left Los Angeles as soon as he could to be with his family in Toronto.

Tristan posted a ton of photos with his mom over the years, it's clear they were very close. He also shared photos of Andrea with his kids -- including his daughter True, who he shares with Khloe Kardashian.

As you know, Khloe and Tristan have two children together, including a baby boy born in August via surrogate.