The Football Association will seek to ensure Nicolas Anelka serves his 'quenelle' suspension wherever he goes next.
West Brom have sacked Nicolas Anelka citing "gross misconduct" after the striker claimed on Twitter he had quit the club.
The 25-year-old was one of the mainstays of Southampton's promotion campaign last season, but he has found first-team opportunites limited since the Saints entered the top flight.
The Belgian has been restricted to just seven appearances in all competitions this season, although he has featured in four games since the turn of the year.
De Ridder, who spent three years with Dutch side De Graafschap before moving to Southampton in the summer of 2011, becomes Dougie Freedman's fifth signing of the transfer window.