Chief Whip Gavin Williamson has been appointed as the new defence secretary after Sir Michael Fallon's resignation.
Sir Michael quit on Wednesday saying his past behaviour may have "fallen short" of the standards expected by the UK military.
He became the first politician to quit following recently revealed claims of sexual harassment in Parliament.
South Staffordshire MP Mr Williamson, 41, said he was "both honoured and excited" by the promotion.
He has been replaced as chief whip by his former deputy, Julian Smith, who will now be in charge of enforcing party discipline in the Commons. Mr Smith's deputy will be Tatton MP - and former GMTV presenter - Esther McVey.
In a statement, Mr Williamson said he was "determined to ensure that the armed forces receive the recognition they deserve for the great work they do, including through the Armed Forces Covenant, and that they evolve both to meet the changing threats that we face and to ensure that they properly represent the modern society that they defend".
He told reporters it was an "immense privilege" to be able to work with Britain's armed forces. He said his priority would be to continue to focus on "countering" Daesh, or so-called Islamic State, and "making sure national security is at the forefront of everything we do".
BBC political editor Laura Kuenssberg said that while Mr Williamson was seen as an effective operator some Conservatives were furious that an MP with no ministerial experience had been promoted to the cabinet.
One senior Conservative told her: "MPs are deeply unhappy he has used the position of chief whip to benefit himself and has deserted his post at such a crucial time".
Asked about claims he lacks ministerial experience, Mr Williamson said: "I've been a minister as chief whip, but it was a little bit quieter when you're chief whip, not so much publicity."
One Tory MP, Sarah Wollaston, told BBC Radio 4's World at One that Penny Mordaunt and Tobias Ellwood had been possible alternatives adding: "These are decisions that are made through patronage. Part of the role of the chief whip is to advise the prime minister about the suitability of the candidate."
But Conservative MP Bob Stewart - a former Army colonel - described Mr Williamson as a "decent, calm man... he's also a very thoughtful man".
"He won't know much about defence but I believe the civil service in the Ministry of Defence, the generals and the Armed Forces themselves won't mind that too much because he's the sort of person that will listen carefully, take advice but then make his own decision."
Battles ahead for Williamson
BBC defence correspondent Jonathan Beale
Gavin Williamson has no military background and has never been a secretary of state. But he is youthful, a rising star and trusted by Number 10.
He arrives at the Ministry of Defence at a difficult time. His first challenge will be to try to stave off more defence cuts. The Cabinet Office is currently carrying out a defence and security review which is due to report by the end of the year.
All three services have been asked to put forward options for cuts. Although there has been a modest rise in the MoD's budget, it has still got to make more than £20bn of efficiency savings.
There's also pressure to fund an increase in pay for the armed forces. They've been struggling with both recruitment and retention. That also hasn't been helped by a political reluctance to put troops in harm's way - something they train for.
There is still unfinished business against so-called Islamic State and Gavin Williamson will be the man who now oversees the RAF airstrikes. Few inside the military or the MoD will know much about him - even fewer on the international stage.
The prime minister's official spokesman said Mr Williamson was not involved in reshuffle discussions, and said he was "an excellent and hard-working chief whip and the prime minister thinks he will make an excellent defence secretary".
"The PM is confident in the operation of the whips' office during her premiership."
In his resignation letter, Sir Michael Fallon said a number of allegations that had surfaced about MPs, including himself, had been false, but added: "I accept that in the past I have fallen below the high standards that we require of the Armed Forces that I have the honour to represent."
Who is Gavin Williamson?
- 41 years old
- Married with two children
- Born near Scarborough
- Ran a Staffordshire pottery business before entering Parliament
- MP South Staffordshire since 2010 and was Parliamentary aide to ex-PM David Cameron
- Chief whip - in charge of party discipline - since July 2016
- Keeps a pet tarantula called Cronus on his desk at Westminster
- Also a patron of the World Owl Trust and has built a house for hedgehogs in his garden
Foreign Secretary Boris Johnson said Sir Michael had a record to be proud of as defence secretary.
"I've known Michael for many years," he said.
"He has been a great colleague and a great defence secretary."
Asked by the BBC if his own behaviour had always been of a standard expected of cabinet ministers, Mr Johnson replied: "You bet."