Chelsea's Romeu loaned to Stuttgart for the season
The 22-year-old, who made 16 Premier League appearances in his first season at Chelsea, signed a three-year contract at Stamford Bridge last month.
A knee injury has hampered Romeu's development and he will now look to gain further first-team experience with a side who finished 15th in the German top flight last term.
"I look forward to the challenge, to a new country and new people, too, which brings me further in my development," Romeu told Stuttgart's official website.
Stuttgart coach Armin Veh, who returned to the club for a second spell in May, also signed forward Daniel Ginczek and midfielder Adam Hlousek from Nuremberg in June.
Romeu has represented Spain at under-21 level and played for his country at the 2012 London Olympic Games.