Reach, 20, will fiil the gap in the squad left by Kyel Reid, who was ruled out for at least six months after tearing his anterior cruciate ligament against Sheffield United last Saturday.
The England U20 international spent the first half of the season at the Bantams' League 1 rivals Shrewsbury and featured against them in the Shrews' 2-1 defeat at Valley Parade in September.
The imposing wide-man, who made 22 appearances for the Shrews, scoring three goals, has signed initially until February 22, with a view to extending the deal after that.
Bantams boss Phil Parkinson told the club's official website: "Adam will bring some extra pace and athleticism to the squad.
"He did well while on loan at Shrewsbury earlier this season and we are delighted to bring him here to the football club."
Reach made 20 appearances for Boro last season, scoring twice.