The young striker has been at the Boro since he was 9 years old and he signed his first professional contract for the club in October 2022.
He made his first-team debut in the last game of last season against Cheltenham Town where he came on as a late substitute.
The move sees Smith link back up with Rebels boss Chris Agutter who worked with the youngster in his stint as Academy Coach at Stevenage in 2021/22.
Agutter said, “We are delighted to welcome Harrison to the club, we also thank Stevenage Football Club for allowing us to bring H in on loan.
“Having worked with Harrison previously I know he will add more firepower to our attacking unit which will go some way to filling the void left by some of our summer departures.
“Harrison is an all-action, big and powerful centre forward who will not only add goals but will also help bring others in to play”.
The 19-year-old has been in and around the group during pre-season and he’s spoken about his excitement about joining the club on loan.
“It’s good to be here,” said Smith, “I’m excited to play for the club and experience some men’s first-team football.
“I’ve learned so much from Agi before. I know his coaching style and how he works and I really got on with him. I felt like I was playing very well when he was at Stevenage”.