He will start training with the club today and initially have until Christmas to try and earn a deal at Emerald Headingley.
Donaldson, 27, was released by Hull KR at the end of the season following a four-year stint at KCOM Craven Park.
He has been looking for a new club ever since and training at a gym in south Leeds with ex-Rhinos back-row Chris Clarkson – also let go by the Robins – in a bid to maintain his fitness.
The industrious ex-England Schoolboys international is well-respected for his work-rate and positive attitude and will now bid to show Rhinos head coach David Furner he could be a useful addition for 2019.
One of the best youngsters in his age group, Cumbrian Donaldson was signed by then Bulls boss Steve McNamara and made his Super League debut for Bradford in 2009 when still aged just 17 .
Originally a loose forward, he has overcome a series of serious knee injuries and played 21 games for the Robins last term.
London Broncos have recruited Australian forward Luke Yates, 23, from NRL side Newcastle Knights.
The prop/loose forward becomes the fifth signing of Danny Ward, the ex-Leeds front-row who guided London to promotion via the Million Pound Game in his first season in charge.
Yates joins on a one-year deal and played 25 times for Knights.