6th October 2017, 18:25 | StHelens
The 21-year-old, who spent the second half of the 2017 season with Leigh Centurions, has signed a two-year contract with the Broncos and will be a replacement for Australian centre Adrian Purtell.
London coach Andrew Henderson said: "We identified and signed Matt some time ago when we learnt Adrian Purtell was returning home to take up a coaching opportunity.
"It was important that we added quality, as good centres are hard to find these days in the current market and with the budgets we work to. I believe we have got a good one here. He is still young but brings Super League experience with him after playing at that level with Saints and Leigh."
Fleming, who was in the Centurions side that lost the Million Pound Game to Catalans Dragons last Saturday, said: "London is a club on the up and I really like what they are all about.
"I believe that I can fit in and play the level of rugby league that is needed for the Broncos to make it back to the top level and that we can bring Super League back to the capital."