The former Denmark international only arrived at the Parkstad Limburg Stadion in December 2013 but was not able to preserve the club's top-flight status as they finished bottom of the table.
Roda announced the departure of Tomasson on Monday and also confirmed technical director Leon Vlemmings will leave his role.
"Roda JC take immediate leave of head coach Jon Dahl Tomasson," read a statement on their official website.
"The Dane was presented in December 2013 as a successor to the laid-off Ruud Brood.
"The disappointing sports results during the past six months and the corresponding relegation to the Jupiler League underlie the termination of cooperation with Tomasson."
Roda collected just 11 points during Tomasson's short spell at the helm.
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