Their letter, published in the internet edition of the daily paper Le Matin, congratulated him for having fled increased taxes in France.
"If it so happens that your stay in Belgium is disagreeable, you should know that we will always fight to ensure that Switzerland defends its full fiscal sovereignty," it added.
The letter, dated December 22nd, was signed by Philippe Nantermod and Maurus Zeier, who co-chair the party's youth wing.
What he saved in taxes by a move to Switzerland "you could give to Swiss cinema," they suggested.
The PLR holds 30 seats in the 200-seat parliament.
The 63-year-old French star threatened last week to give up his French passport and take up Belgian citizenship to protest the Socialist government's new tax hike on the rich.
He made the threat after French Prime Minister Jean-Marc Ayrault called him "pathetic" for making Belgium his formal place of residence, a move meant to help the actor avoid the French tax.
Swiss commentators have argued that Depardieu's choice of new residence country is proof that Switzerland, already home to a number of wealthy French tax exiles including rocker Johnny Hallyday, is no longer Europe's tax haven of choice.