In a 7-minute televised speech Tuesday, the 61-year-old ruler said after 18 years in the position, it is time for him to hand over power to the next generation, Gulfnews.com said.
"As I address you today, I declare that I will hand over the reins of power to Sheik Tamim bin Hamad al-Thani and I am fully certain that he is up to the responsibility, deserving the confidence, capable of shouldering the responsibility and fulfilling the mission," al-Jazeera quoted him as saying. "God Almighty is aware that I had not desired power for the sake of power; nor endeavored to rule for personal motives. Yet it has always been the nation's interests and that interest has dictated that we lead through a new chapter."
Tuesday was declared a public holiday.
The new emir was educated in England and graduated from the Royal Military Academy at Sandhurst.
Tamim has been groomed for the position since 2003, al-Jazeera said. He is deputy commander of the armed forces, head of the National Olympic Committee and chairs the Qatar 2022 Supreme Committee, which will host the 2022 FIFA World Cup.
Monarchs in the Gulf region rarely step aside, The Wall Street Journal said. One key question is whether the prime minister, Sheik Hamad bin Jassim bin Jaber Al Thani, will also leave office.
U.S. President Barack Obama released a statement welcoming Qatar's new ruler.
"I extend my best wishes to His Highness Sheikh Tamim bin Hamad Al Thani as he assumes his new role as the Amir of Qatar," Obama said. "Qatar is an important partner of the United States, and we look forward to further strengthening our cooperation in the years ahead. I also extend my appreciation to His Highness Sheikh Hamad bin Khalifa Al Thani for his friendship and leadership. The United States looks forward to working with Sheikh Tamim to deepen the ties between our two countries, and to continue our close partnership on issues of mutual interest."