He wields enormous power and leads a war against rebels in neighbouring Yemen for his father King Salman, but Saudi Arabia's defence minister is still in his early 30s.
Just weeks after he was appointed to the key post in January, Prince Mohammed bin Salman assumed huge responsibility when the kingdom sent its armed forces into conflict.
The young prince has overseen nearly a month of air strikes by a Saudi-led regional coalition against the Yemeni rebels. Eight Saudi troops have also died in skirmishes along the border.
Prince Mohammed, part of a new generation of Saudi rulers, also heads the royal court, is special adviser to his 79-year-old father and sits on two key political and economic councils.
"He is the strong man in Saudi Arabia," a Western diplomatic source said. "Look what this man is controlling," including access to the king.
"He oversees everything important which is going on in this country."
The exact age of Mohammed, who sports a full dark beard, is uncertain.
A biography from the Saudi embassy in Washington does not give his birthdate. Analysts and local media have reported various ages but none higher than 35.
As chief of the royal court he holds "a position of immense power in an absolute monarchy," wrote Bruce Riedel, a former Central Intelligence Agency officer who directs the Brookings Intelligence Project in Washington.
He said Prince Mohammed oversees the kingdom's security in conjunction with Interior Minister Prince Mohammed bin Nayef, who headed a crackdown on Al-Qaeda a decade ago, and Prince Miteb bin Abdullah -- minister of the National Guard, a parallel army.
According to a biography from the MiSK Foundation, which Prince Mohammed established for youth development, he had "a professional career of 10 years" and was active in business and philanthropy before entering public service.
In 2009 he became special adviser to his father who was then the governor of Riyadh, before heading his court from 2013 after Salman was named crown prince.
In April last year Prince Mohammed became a state minister and cabinet member, before his appointment as defence minister and chief of the royal court on January 23, the day Salman became king upon the death of his predecessor Abdullah at the age of about 90.
"He has a reputation for being aggressive and ambitious, as might be expected," Riedel said of the prince, whose father also served as defence minister.
The war in Yemen, which the UN says has left hundreds dead and thousands wounded, has raised Prince Mohammed's profile even higher.
