He was encouraging the opposition to take up their seats in parliament, which they have been boycotting since an election last year that they said was marred by fraud.
"In Thailand, (coup leader General) Prayuth Chan-ocha received a royal decree from the King so he can work legally," Hun Sen said Thursday, according to the Phnom Penh Post.
"Here, the National Assembly (also) obtained a royal decree for the convening of parliament," he said.
The premier called the opposition Cambodia National Rescue Party "stupid" for refusing to take their seats.
"A royal decree is the highest legitimacy above all others," he said.
DPA