The midfielder from Portslade completed a permanent move to Stevenage.
He will come under the command of Stevenage boss Alex Revell, the striker who forged a Football League career after Albion signed him from Braintree.
Knight had been lined up for a loan move to Crawley late in the summer window but that fell through.
The expectation had been for a temporary move in January but he has instead completed a full transfer.
Knight posted photos of himself in Albion kits from down the years on social media with the caption: “Thanks for the last 12 years. All the best.”
The now 20-year-old joined the academy as an under-12, progressing through the age groups before making a brief Premier League debut in November.
Albion sporting director Jason Ayto said: “This is a great move for Joe.
“He wants to go and play and now has the opportunity at a club pushing for promotion in League One.
“He is a credit to our academy and all those coaches and staff who have worked with him over the past decade. We wish him the very best of luck for the future.”
Further outgoing business among the under-21s is expected ahead of Monday’s transfer deadline.