The 33-year-old, who lists Bayern Munich, Liverpool and Marseille among his former clubs, has agreed a deal to join the Championship side for the remainder of the season, having left West Ham last year.
Diarra captained France in a match at the World Cup in South Africa five years ago and could come into Guy Luzon's side for Tuesday's Championship clash with Derby County.
Head coach Luzon told the club's official website: "He will add some height, experience and, most of all, quality to the midfield.
"We lost Milos [Veljkovic] to injury and Johnnie Jackson is out for a month, so he will help us to bring results and more points.
"It's a coup for us that we can attract a player with so much international experience and I'm sure he will be fantastic for us."
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