Sheikh Sabah al-Ahmad al-Sabah has been sworn in after his unanimous election by parliament as Kuwait's new ruler.
Sheikh Sabah is seen as a reformist leader
Sheikh Sabah, who has been prime minister since 2003, is the oil-rich Gulf state's third ruler in two weeks.
He succeeds Sheikh Saad al-Abdullah, who was removed by legislators on health grounds last Tuesday after just nine days in power.
The appointment brings to a close a leadership crisis which saw parliament take on an unprecedented role.
It was the first time a Gulf ruler had been deposed by an elected body.
Legislators voted 64-0 on Sunday morning to appoint Sheikh Sabah, who is in his mid-70s and served as foreign minister for 40 years.
Analysts say he is a reformist minded statesman who has pushed ahead with enfranchising women and economic liberalisation.
The confirmation brings to an end a succession struggle within the ruling al-Sabah family following the death of Emir Jaber al-Ahmad in January.
A BBC correspondent in the Gulf says Kuwaitis watched in amazement as members of the ruling al-Sabah family quarrelled about the succession.
The emirate controls about 10% of the world's proven oil reserves and is a key US ally in the region.