- Although Maatsen is moving to Germany for now, he has also signed a contract extension until 2026 with Chelsea.
- Dortmund's deal for Maatsen is the club's second loan signing in as many days.
Borussia Dortmund have signed Chelsea defender Ian Maatsen on loan for the rest of the season.
The 21-year-old has made 15 appearances for Chelsea this term, but just three of those have been starts.
The Netherlands Under-21 international spent last season on loan at Burnley but rejected a permanent move to Turf Moor in August.
Although Maatsen is moving to Germany for now, he has also signed a contract extension until 2026 with Chelsea.
Speaking on Friday ahead of his side's trip to Darmstadt on Saturday, Dortmund coach Edin Terzic backed Maatsen to make a positive impact at the club.
"We hope that Ian will quickly fit into the group and become an important factor for us," he said.
Maatsen had hoped to break into Mauricio Pochettino's starting XI this season but started just once in the Premier League, with two further starts in the EFL Cup.
The left-back, who can also play on the left wing, joined Chelsea from PSV Eindhoven in 2018.
Maatsen scored four goals and added six assists for Burnley last season and helped the Clarets to promotion to the Premier League.
Vincent Kompany's side had a £31.5million bid accepted by Chelsea last summer, but Maatsen opted to remain at Stamford Bridge.
Dortmund's deal for Maatsen is the club's second loan signing in as many days, with Manchester United winger Jadon Sancho returning to the club on a six-month loan on Thursday.
"We are happy that Jadon is finally with us," said Terzic.
"He had a lot of fun on the training pitch. He is healthy, he has trained a lot in the last few weeks. We will use the session again today and see how far he is."