Hun Sen stepped down in August after nearly four decades of rule, passing the baton to his eldest son Hun Manet.
In February his ruling Cambodian People's Party (CPP) won 55 seats -- including his own -- out of 58 in the senate election, with the opposition Khmer Will Party taking the remainder.
On Wednesday senators unanimously voted to approve Hun Sen as president of the senate after King Norodom Sihamoni convened its first meeting.
The position allows Hun Sen to act as acting head of state when the king is overseas.
Addressing the senators after the vote, Hun Sen thanked them for the "honour".
"It is my first time to sit on such a high chair," he said, saying he would use the second-highest ceremonial role -- only superseded by the king -- to further Cambodia's international diplomacy.
There are 62 senators, with 58 elected in February by 125 MPs and more than 11,000 local administrators, mostly members of Hun Sen's party.
King Norodom Sihamoni appoints two senators, while the National Assembly appoints two others.