Offerdahl, 27, missed out on a move to Championship rivals Bradford last winter due to a recurring injury, but will now make the move to the UK subject to a successful visa application.
He said: “I’m very pleased to be joining the Broncos, it’s a club I’ve been wanting to sign with for a while now. I can’t wait to get there and start training.
“The Broncos are a full time team who want to be back in the Super League and I want to be a part of that. I’ve played in a few different countries but never lived in London which is a very appealing city to live in.
“The challenge of helping London get promoted is something that excites me. It’s a big draw card to any player looking to join a Championship team. The Super 8’s idea is innovative and I’m looking forward to watching the upcoming series.”
Broncos coach Andrew Henderson said: “I’m really excited to be bringing Mark to the club, as a player he predominately plays prop but can play on an edge as well so gives us some versatility.
“After speaking to Mark I know he appreciates this opportunity, and has the ambition to help us get back to Super League.
“These are the type of player I want at this club, people that have the hunger to be better and want to achieve things with the club. I’m really looking forward to what Mark will bring to the team.”