Moffat, who left Netherdale to join Castres in the summer, scored six tries in 25 appearances for Borders last season.
The Scot will re-join the squad this week ahead of their next Celtic League clash against Munster at Netherdale on March 16.
The 29-year-old said: "My move out to France hasn't brought as much game time as I had hoped for, so I'm really looking forward to coming back to the Borders and getting stuck in again.
"Having helped take the Reivers into the Heineken Cup last season, it's very frustrating having to sit here and watch it on TV."
Reivers head coach Steve Bates added: "We were disappointed to lose Stuart at the end of last season but we understood his decision to experience French rugby at this stage of his career.
"We have been looking to strengthen our squad, especially in the backs, so we are happy to welcome him back."