He will replace HR McMaster.

US President Trump made the announcement on Twitter on Thursday.

He 'thanked' HR McMaster for his service and said he will always remain his friend.

"I am pleased to announce that, effective 4/9/18, @AmbJohnBolton will be my new National Security Advisor. I am very thankful for the service of General H.R. McMaster who has done an outstanding job & will always remain my friend. There will be an official contact handover on 4/9," Trump tweeted.

Meanwhile, Bolton said it will be an honour for him to serve the nation.

Bolton told Fox News' “The Story” Thursday evening, “I didn't really expect that announcement this afternoon, but it's obviously a great honor. It's always an honor to serve our country and I think particularly in these times internationally, it's a particular honor.”

US Vice President Mike Pence welcomed the appointment and thanked McMaster for his service to the country.

Pence tweeted: "I will always be grateful to my friend Gen. H.R. McMaster for his service as @POTUS' Nat Sec Adv & congratulate him on a job well done. He departs w/ our highest respect for his service to this Admin & his heroic service on & off the battlefield. Thank you H.R. and God bless."

Welcoming the new National Security Adviser, he said: "Congratulations to new National Security Advisor John Bolton. @AmbJohnBolton is a highly respected American patriot who will bring decades of experience to our national security team. With him as Nat Sec Adv, @POTUS will continue to keep America strong and secure." 

US Ambassador to the United Nations Nikki Haley also thanked HR McMaster for his service to the nation.

"Thank you to Lieutenant General HR McMaster for your service and loyalty to our country. Your selfless courage and leadership has inspired all of us. Most of all, thank you for your friendship," she tweeted.

Trump Image: Facebook

Bolton Image: John Bolton/Twitter