A popular and colourful figure in France, the 63-year-old Depardieu is the latest wealthy Frenchman to seek shelter outside his native country after tax increases by Socialist President Francois Hollande.
Prime Minister Jean-Marc Ayrault described Depardieu's behaviour as "pathetic" and unpatriotic at a time when the French are being asked to pay higher taxes to reduce a large national debt.
"Pathetic, you said pathetic? How pathetic is that?" Depardieu said.
The star recently bought a house in Nechin, a Belgian village near the border with France, where 27% of residents are French.
Depardieu, who inquired about procedures for Belgian residency, said he was handing in his passport.
Culture Minister Aurelie Filippetti said she was outraged by Depardieu's letter, adding that he had for years been supported financially by public money for the film industry.
"When we abandon the ship and desert in the middle of an economic war, you don't come back and give morality lessons," she said.
Mr Hollande is pressing ahead with plans to impose a 75% supertax on income more than €1 million.