Lovegrove, 27, was released from the final year of his deal at the KC Lightstream Stadium on compassionate grounds, and the move to London will enable him to move closer to family members.
He said: “Joining the Broncos is a great move on and off the field.
“On the field, I am excited by the new challenge of helping the Broncos regain Super League status, while continuing to promote and develop rugby league in the capital city. Head coach Joe Grima sold the club superbly with his ambitious and inspirational attitude.
“Off the field, my partner Libby has been extremely supportive during my eight years at Hull KR and we now look forward to fostering relationships with her family and friends in the London area.”
Lovegrove has made 159 appearances for Hull KR since joining from St George-Illawarra in 2007, and is a former Australian schoolboy international.
Coach Joey Grima said: “Rhys is another great addition to our squad for 2015 and again shows the calibre of player we are attracting with our vision for the club moving forward.
“We’ll welcome Rhys to the club for pre-season training in November as a mature and experienced player who will work well with our younger players to help develop them whilst also concentrating on his game.”