The Keeping Up With The Kardashians star, 36, gave birth to her third son in December and has kept him relatively under wraps, sharing just one picture of him previously.
Alongside the new photo, Kourtney wrote: "My RAD little bunny."
Kourtney and long-term partner Scott Disick, 31, are also parents to son Mason, five, and two-year-old daughter Penelope.
While Kourt is happy to post snaps of Mason and Penelope to social media, and they've featured heavily on the family family show, it seems that she and Scott are trying to keep their newest baby out of the spotlight.
Showing Reign off to the world for the first time a month ago, Kourtney posted a snap of him laying on a fluffy white rug.
She wrote: "My little turtle dove angel baby boy Reign Aston Disick."
Earlier this year Scott discussed what it was like introducing Reign to his older siblings, admitting he was surprised at their lack of jealousy.
He told US Magazine: "I really would have thought the other two would have been jealous.
"They're so accommodating and welcoming – I don't think I would have been that way if I had a sibling. I can not get over how much affection the other two have shown the baby."