The duo will be on loan until the end of the season before taking up long-term deals from 2020.
Back-rower Hadley, 26 and hooker Litten, 21, trained with their new team-mates for the first time on Monday.
Both have signed a three-year contract to begin next year.
Hadley has represented England at academy and Knights level and had a previous spell on loan at Wakefield Trinity.
He has made 89 appearances for Hull since 2013 while Litten has played 19 times for the Black and Whites over the last three seasons and also featured on loan with Doncaster.
Hadley said: "It has been quite a frustrating year for me really. On the back of the last couple of years I felt like I’d improved on loan at Wakefield and I’d come back to Hull and started playing well in 2018, but this year hasn’t gone to plan. However, I see this as a great opportunity at Hull KR with so much still to play for in our season.
“The record of Tony [Smith, Robins' head coach] speaks for itself.
"People I’ve spoken to have nothing but good words to say about him. I’ve had a brief chat with him and I’m looking forward to meeting up with the rest of the boys on Monday."
He added: “I know Crooksy [Ben Crooks] from our time at FC as well as Robbie Mulhern and Chris Atkin from the Knights tour last year, but I am looking forward to that change of scenery which I feel I needed.
I’m excited to meet new team-mates and new faces.
“If Tony wants someone who can play big minutes, put in some hard graft and bring a bit of skill to the table, then that’s what I can do.”
Litten said: “I’ve been playing at Doncaster over the last few weeks and the opportunity to join Hull KR would have been silly to pass up on. I’m really looking forward to getting some first-team experience.
“I want to keep learning and adding things to my game, I just want to keep on improving myself.
“I’ve been watching Matt Parcell on TV recently and he’s great to watch. Hopefully I can bring similar sorts of things to the team that he has in the hooker position.
“I want to do my best for Rovers now and I can’t wait to get started. It’s good that I’m coming across with Dean because we’re pals but I know a lot of the younger lads like Owen Harrison, Adam Rooks and the rest which is good.”