The 22-year-old spent last season on loan from parent club Wolverhampton Wanderers, scoring seven goals in 42 appearances.
The Royals were reportedly keen on a return for the Wales youth international but Alex Revell's Boro have confirmed the acquisition hours before this evening's deadline.
Due to the FIFA Club World Cup, the summer window has been split into two and will reopen on June 16, remaining open from then until 7pm on September 1.
He's here. ⚡️ pic.twitter.com/V9Oej4AOQn
— Stevenage FC 🔴⚪ (@StevenageFC) June 10, 2025
Clubs may announce signings between the two windows, but governing bodies will not fully ratify the deal in that time.
Stevenage finished 13th last season, drawing both fixtures against the Royals, and have made three signings.
Campbell recorded a goal contribution in the 1-1 draw at the Lamex Stadium fixtures, setting up Harvey Knibbs for an equaliser.
Noel Hunt awaits his first signing of the summer, but new owner Rob Couhig believes it has 'the potential to be spectacular.'