The 25-year-old Dutchman had a brief spell at the Liberty Stadium three seasons ago, making himself an immediate fan favourite by scoring the winner in a south Wales derby win over Cardiff.
Swansea have tried to bring the striker back to the Liberty Stadium on several occasions since and a deal until the end of the season has now been agreed, pending Premier League approval and international clearance.
Emnes has scored just once this term for Boro but attracted interest from Swansea after moves for Tottenham's Nacer Chadi, Norwich's Anthony Pilkington and Tom Ince from Blackpool came to nothing.
A statement on the Swansea website read: "Marvin Emnes is set to return to Swansea City after agreeing a loan deal from Middlesbrough that is now subject to Premier League and international clearance.
"Emnes, who proved a popular figure with the squad during his earlier spell with the Swans, knows the set-up and style of play well.
"That familiarity should enable him to settle in immediately as the Swans embark on a busy schedule in the Barclays Premier League, Europa League and FA Cup.
"The former Sparta Rotterdam star will provide competition out wide for manager Michael Laudrup, while he will also bolster the Great Dane's options up front."