What do we know about him Herbert Raymond McMaster?
McMaster seemed won praise after his appointment. He is less controversial than Lieutenant-General Michael Flynn famously lasted less than a month on the job after being sacked for contacts with the Russian government.
McMaster, 54, is a career army officer and prominent expert on counterinsurgency warfare. He is known for his criticism of the US military's handling of the Vietnam War and his own service as a commander in northern Iraq in 2005.
Trump described McMaster as "man of tremendous talent and tremendous experience".
What is the National Security Advisor's role?
The position is a crucial engine for the smooth functioning of government.
The National Security Advisor manages hundreds of staff members, arbitrates between government departments, balances foreign policy and military policy and ensures the president's national security agenda gets implemented.
Powerful figures, such as former general Colin Powell, have held the post in the past.
What should we expect from McMaster?
McMaster's experience in Iraq's Tal Afar is likely to come in useful as US and allied forces attempt to retake nearby Mosul from the Islamic State group.
"He, in a sense, believes in speaking truth to power and he has been very critical of the fact the the Bush administration had gone into Iraq with very little planning," says Laleh Khalili, a professor at Soas in London.
But she has misgivings.
"What is less reassuring is that McMaster is known as one of the gurus of counterinsurgency doctrine. It means that he is a very intelligent proponent of pacification and unconventional resistance in many instances," Khalili told RFI. "Because he is a counterinsurgent, he believes in boots on the ground or large numbers of military officers and soldiers in order for such pacification to proceed."
Trump has surrounded himself with men from the military. Is that unusual?
It is. On Monday, Keith Kellogg, a retired three-star general who had served as acting national security advisor was also appointed as.National Security Council chief of staff.
"There is a particular way in which Trump displays particular forms of masculinist aggression," says Khalili. "Him being enamoured of these generals is a symbol of the hypermasculine, militaristic type of discourse that has been characteristic of his campaign."
Whether the respected but controversial McMaster is able to influence Trump on matters such as his ban on residents of seven Muslim-majority countries, which has been struck donw in court, remains to be seen.
Staffers in the White House, fear that the National Security Council is currently being bypassed by political aides like far-right strategist Steve Bannon.
"It's also the civilians surrounding Trump that are worrying," notes Khalili. "The combination of the civilians, that are clearly the ideologues of the extreme right wing, with international realistic military officers makes for a probably volatile administration."