Grant Shapps vowed to continue support for Ukraine as he was announced as the new defence secretary on Thursday.
The Prime Minister carried out a mini reshuffle at the heart of his Government after Ben Wallace officially resigned the post he has held for four years.
Claire Coutinho, previously an education minister, will take over as energy secretary.
Mr Shapps paid tribute to his predecessor and added: “I am looking forward to working with the brave men and women of our armed forces who defend our nation’s security. And continuing the UK’s support for Ukraine in their fight against Putin’s barbaric invasion.”
It is the fifth senior Cabinet position Mr Shapps has held in the last 12 months, having also served as transport secretary, home secretary, business secretary and energy secretary.
Mr Wallace announced last month that he would be leaving the Ministry of Defence.
In a letter to Rishi Sunak on Thursday morning, he said: “As I finish my tenure, I can reflect that the Ministry of Defence that I leave is now more modern, better funded and more confident than the organisation I took over in 2019. As well as being active around the world we have also invested in prosperity at home.
“The United Kingdom is respected around the world for our Armed Forces and that respect has only grown more since the war in Ukraine.
“I know you agree with me that we must not return to the days where Defence was viewed as a discretionary spend by Government and savings were achieved by hollowing out.”
He added that his dedication to the armed forces had come at a “personal toll to me and my family”.
“After much reflection, I have taken the decision to ask that I be allowed to step down,” he said. “I won my seat in 2005 and after so many years it is time for me to invest in the parts of life that I have neglected, and to explore new opportunities.”
The PM said Mr Wallace had served his country with “distinction”.
Mr Sunak added: “You have been a vigorous advocate for the defence of the realm.”
Mr Shapps was the surprise frontrunner for the job on Thursday morning.
He visited Ukraine last week and tweeted: “What I saw in Kyiv this week was a people resolute in their defiance against Putin’s tyranny and Russia’s aggression.”
Former PM Boris Johnson said he was “sad to see” the departure of Mr Wallace but “Grant Shapps is an excellent choice to succeed him”.
Mr Wallace, who served under three prime ministers in his current role, played a key part in the UK’s response to Russia’s invasion of Ukraine and was a close ally of Boris Johnson.
Popular among Tory members, he was at one time seen as a potential leadership contender.
He ruled himself out of the running for the Conservative leadership last year despite being an early frontrunner in the race to replace Mr Johnson.
His successor will take on the high-profile role as the war in Ukraine continues.
A Labour source said: “The outgoing defence secretary has shown real leadership in supporting Ukraine from the start and Labour will continue to stand shoulder to shoulder with the government in support of the people of Ukraine until the war against Russia is won.
“We have to stay the course and see this through. While Ben Wallace deserves credit for his support of Ukraine’s armed forces, his record on British armed forces has been poor, with cuts and procurement failures the order of the day. The new defence secretary needs to get a grip, boost British forces and give them the resources, kit and accommodation they deserve.”