The Emir of Qatar, Sheikh Hamad bin Khalifa Al Thani, is to hand over power to his son, Sheikh Tamim bin Hamad Al Thani.
A palace statement said that Qataris would be invited to swear allegiance to the new emir on Wednesday.
Sheikh Hamad is due to address the nation at 8am on Tuesday, which will be a national holiday.
Rumours had been circulating for days that Sheikh Tamim, 33, was preparing to succeed his father.
al-Jazeera TV reported that Sheikh Hamad, 61, had told a meeting of the "ruling family and top advisers" of his decision on Monday.
The BBC reports a cabinet reshuffle is also expected as part of the changes.
Qatar has been dominated by the Al Thani family for almost 150 years.
Sheikh Hamad seized power from his father Sheikh Khalifa in a bloodless coup in 1995, with the support of the armed forces and cabinet.
The BBC reports he has introduced some political and economic liberalisation, and made Qatar a major player in regional diplomacy in recent years.