The Prime Minister’s comments came before a statement on the case by Foreign Secretary Jeremy Hunt.
Mrs May told MPs: “I am sure the whole House will join me in condemning the killing of Jamal Khashoggi in the strongest possible terms.
“We must get to the truth of what happened.”
The Prime Minister’s remarks in the Commons followed former foreign secretary Boris Johnson urging a robust response to Saudi Arabia over Mr Khashoggi’s death.
Britain should “refuse to turn a blind eye” to the murder of the dissident journalist and pressure Saudi Arabia into ending the brutal civil war in Yemen, Mr Johnson said in his Daily Telegraph column.
The former foreign secretary drew parallels between the Istanbul killing and the Novichok attack in Salisbury in March, calling them “state-sponsored plots” designed to “send a terrifying public warning” to opponents.
The Gulf kingdom admitted on Friday that Washington Post columnist Mr Khashoggi, a vocal critic of Riyadh, was killed at its Istanbul consulate but claimed he died after a fight broke out.
Mr Johnson said “we cannot just let it pass”, and, while the UK has crucial trade and security links with the Gulf State, “the UK and the US must lead other countries in holding Saudi Arabia properly to account”.
He added that pressure should be applied to force the Saudi leader to put an end to the Yemen war, which has been heavily criticised for human rights abuses, including high civilian casualties.
Mr Johnson wrote: “The Saudi Crown Prince has the chance to be the godfather of peace - and to save his country from the threat of missile attacks.
“Now is the time for him to act. Britain and Saudi Arabia have interests in common and historically friendly relations.
“But, at moments like this, it is the job of a friend to tell the truth; and the truth is that the killing of Khashoggi was a barbaric act to which we in Britain refuse to turn a blind eye.”
Turkish government sources have claimed that Mr Khashoggi was tortured and murdered by a hit squad flown in from Riyadh.
The Saudis initially dismissed the allegations as baseless, without providing an explanation as to how the Washington Post columnist disappeared after entering the consulate on October 2.
A number of Saudi nationals have since been arrested, while deputy intelligence chief Ahmad al-Assiri and Saud al-Qahtani, a senior aide to the Crown Prince, have been dismissed, state TV reported.
Saudi Arabia is the UK’s key ally in the region and also a significant trading partner. Britain rolled out the red carpet for Crown Prince Mohammed Bin Salman in March.
Labour’s leadership seized on the events in Istanbul to call for arms sales to Saudi Arabia to be halted.
In the Commons, Jeremy Corbyn said he was pleased Mrs May had condemned the “horrific murder” of Mr Khashoggi but added “condemnation is not enough” from the Government.
“Will they now end arms sales to Saudi Arabia?” he asked.
Shadow foreign secretary Emily Thornberry told BBC Radio 4’s Today: “These are supposed to be our friends. It’s up to them to change their behaviour, it’s up to us to stand up to them and it is up to us to make it clear that we do not accept this behaviour and they need to change their ways.”