Holden, who was part of Walsall's 22-man squad which made the trip to Poznan in Poland, has enjoyed a successful trial spell with the team, making several pre-season appearances as he looked to ear a move to the Midlands.
The 21-year-old Northern Irishman started his career with hometown club Derry City before moving to Ashton Gate in August 2017. He went on to enjoy a loan spell with Barrow and also played six League One matches on loan at Rochdale last season.
Speaking via the club's official website, Rory said; "I’m buzzing to be here and to finally get the move over the line. I am a winger who likes to get on the ball and create chances for other players and try and chip in with a few goals.
"The week away with the group was perfect for me. The boys really helped me settle in because they’re a really open and honest group. It is the perfect time for me to come out on loan to help me develop as a player.
"The manager wants me to go out on the pitch and just express myself, gain some experience in the Football League and to try and excite the fans."