Girona are a sister club to City and took five players on loan from the Premier League champions last season.
One such player, Aleix Garcia, will also spend the 2018-19 season at the club, after helping them to a 10th-placed finish last term.
Roberts joined City from Fulham in an £11 million deal in 2015, with the club keen to add to their homegrown quota.
However, he has played just three times for the first XI, and once in the Premier League, as he made a four minute cameo in a 4-1 defeat to Tottenham.
An impressive spell at Celtic, where he made 78 appearances and scored 18 goals, has enhanced his reputation, nevertheless, and he will hope to enjoy similar success in La Liga.