Roda finished bottom of the 16-team standings, having been a fixture of the premier division since 1973.
"The disappointing sporting results of the last half-year and the relegation are the reasons for ending our co-operation with Tomasson," the club said on its website.
Tomasson, 37, joined Roda in late December after a first coaching job in charge of Dutch second division club Excelsior.
He won 112 caps for Denmark and scored 52 goals between 1997 and 2010 and competed in two World Cups.
He also played for AC Milan in the 2005 Champions League final against Liverpool in Istanbul coming on as a late substitute and scoring in the penalty shoot-out which Liverpool won to lift the European Cup after trailing 3-0 at halftime.
His other clubs included Heerenveen, Newcastle United, Feyenoord, VfB Stuttgart and Villarreal.
(Reporting by Mark Gleeson; Editing by Mike Collett)
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