She had worked on the show for two years and had been in Virgin Media for two decades.
And of course, rumours are already swirling about who could step into the hot seat.
One famous singer is the favourite to step into Karen’s very stylish shoes.
According to Ladbrokes, Una Healy is favourite with odds at 3/1 to join The Six O’Clock Show full time.
Hot on her heels is Brian Dowling at 4/1 and Roz Purcell at 5/1.
Other names being on the list include James Patrice (8/10), Zeinab Elguzouli (10/1), Maura Higgins (14/1), and Kathryn Thomas (16/1).
Nicola McGeady of Ladbrokes said: “Karen Koster’s shock departure from The Six O’Clock Show has left fans reeling and following mounting interest, we have opened the betting on who will join Greg O’Shea as the new co-host of the Virgin Media show.
“While it’s still early days, Una Healy is the current frontrunner to land the gig!”
Watch this space!
Karen shared the news that she was leaving the show on Monday night, announcing it at the end of the episode before sharing an emotional post on social media.
“Even though I’ve been thinking about this for months, now that I’ve said it on air, it’s all starting to feel real.
View this post on Instagram
“I’m finishing up on @sixoclockshow next week for the simple reason that I just need to be home more with my family in the evenings. I have had a ball presenting this show for over two years, and I’ve been so lucky to be surrounded by a wonderful production team, producers, gallery crew and of course my work bestie Greg.”
She continued: “Truthfully the only downside to The Six O’Clock show, is that it’s on at 6 O’Clock! And that’s simply a time when I need to be at home, rather than in the studio. But I’ll always be around to mind the mothership should the team need me to jump back into the hotseat.
“Thank you for tuning in and for all the lovely comments I get when I’m out and about. I’m looking forward to sitting down and watching the show as a fan, just like I did before. But for now, it’s time to hand over the baton and start the next chapter. But not just yet, I’ve a few shows in me still.”