The 19-year-old midfielder, who has had previous spells in the Championship, becomes the second Blues player after Kenneth Omeruo to move to Teesside from Stamford BridgeMiddlesbrough are the latest Championship team to secure a loan deal for Chelsea midfielder Nathaniel Chalobah.
The England Under-21 international spent almost three months with Nottingham Forest before returning to Stamford Bridge this week.
Middlesbrough manager Aitor Karanka worked for Chelsea manager Jose Mourinho at Real Madrid and that appears to have helped him to lure 19-year-old Chalobah to Teesside for the rest of the season.
The defensive midfielder enhanced his reputation with a successful loan spell with Watford last term as they reached the play-off final.
His spell at Forest was not as productive, but Karanka will be hoping to get the best out of the teenager as his side aim to push for a play-off place.
Chalobah has not yet made a senior appearance for Premier League high-fliers Chelsea.