Signing a two-year deal, Baker will exit the Roosters at the end of the year with an eye on a permanent spot in Brisbane's rotation.
Playing as a fringe first-grade option at Trent Robinson's Roosters, Baker has managed seven games this year, taking his career tally to 37, however, only once has he managed more than 40 minutes.
The likes of Xavier Willison, Corey Jensen and Kobe Hetherington are already signed by the Broncos for next year, leaving what will likely be a four-way race for the remaining middle third minutes alongside Haas and Carrigan.
Haas is signed until the end of 2024, but no guarantee to be at the Broncos during the second year of Baker's deal, with both rival NRL clubs and rugby union - both at home and abroad - circling.
Broncos coach Kevin Walters said Baker was a welcome addition at the Broncos.
"Fletcher will be a welcome addition to our squad - he is a big frame with plenty of talent and has had a few seasons at NRL level so he knows what it takes to succeed," Walters said in a club statement announcing the signing.
"We've got some really good young middle forwards coming through here at the Broncos and he will add to the strength we have in that position."
Baker will link up with the Broncos in November when pre-season training commences.