Jon Gosselin's Ex-GF Tells Collin She'll 'Always Be Your Mom'

Jon Gosselin's ex Colleen Conrad has offered her unwavering support to his son Collin Gosselin ... and has told him she's "always be your mom" in the wake of his claims against mother Kate Gosselin.

Collin -- whose memoir "In the Shadow of Eight: Surviving the Reality of My Childhood" debuted at No. 1 on the New York Times best seller list for non-fiction -- received a special shout-out from Colleen on Instagram.

She wrote about his book rising to the top of the charts ... "Who would have imagined that all those years ago? I am so incredibly proud of you!"

She added ... "When you came into my life, you had already been through more than any child should ever have to experience."

In the book, Collin writes of Colleen ... "You stepped into my life as the mother I needed most. ... Strong men need strong mothers, and I was so fortunate to have mine."

Colleen -- who was in a relationship with Jon from 2014 to 2021 -- wrote ... "Collin, those words mean more to me than you’ll ever know."

She added ... "No matter how old you get or where life takes you, I will always be your mom. I will always love you and cheer you on."

In response, Collin wrote ... "I love you. I loved that you say you needed me too."

He added further down ... "Sometimes all we need is to know our mom believes in us and it gives us enough faith in ourselves."

We reported earlier ... Collin has accused Kate of horrific abuse when he was a child -- allegations she's dismissed as being part of his mental illness.