The two countries agreed to finalise talks on a multi-billion pound order for 48 Typhoon aircraft, the fighter plane's maker BAE Systems said.
Britain and Saudi Arabia signed commercial deals worth more than $2 billion during Crown Prince Mohammed Bin Salman's visit to London which ended on Friday, a joint statement said, without providing details.
Saudi Crown Prince reviewed with the UK Defense Secretary Gavin Williamson the means of strategic cooperation and developing defense, military and anti-terrorism efforts, on Friday, at the North Holt Air Base.
Saudi Arabia already has 72 Typhoon jets.
The three-day visit will include a second royal audience - dinner with the Prince Charles and Prince William - a briefing with national security officials, and a prestigious visit to the prime minister's country residence.
The deal could save hundreds of jobs at Wharton in Lancashire, where the jets are built by BAe Systems. Will British politicians confront MBS about Saudi Arabia's human-rights record and role in Yemen?
Andrew Smith of Campaign Against Arms Trade said the United Kingdom had already licensed £4.6bn of arms for Saudi Arabia since the Yemen conflict began in 2015.
Mrs May defended the UK's ties with Saudi Arabia earlier this week, saying all arms sales are strictly regulated.
Prince Mohammed's itinerary was largely private, including meetings with bankers, executives and lawmakers, and he did not speak publicly at any point during the trip.
"The fact that we have the head of state of a government that has been operating such a blockade - Saudi Arabia - recently invited to Buckingham Palace and Downing Street while the military.is orchestrating what will potentially become the worst starvation in the last 50 years, I think speaks volumes to another aspect of the problem that I want to highlight - the growing sense of impunity surrounding these crimes against children".
An image of the heir-apparent striding up Downing Street towards May's office was lauded by Saudis on social media as emblematic of his youth, confidence, determination and enthusiasm.
But she has also raised concerns about the humanitarian crisis in Yemen with the Crown Prince during his visit.
Downing Street said the framed document was originally created by Queen Victoria's consul general in Jeddah in 1880. Emily Wither went running with them in Jeddah.