David Cameron and Saudi defence minister Prince Salman bin Abdul Aziz have called for a "concerted" international response to Iran's nuclear ambitions.
Downing Street said the pair also discussed the situation in Syria and agreed that President Bashar Assad had "lost all legitimacy" following his brutal suppression of opposition.
They said the international community needed to focus on achieving a "rapid transition" that reflected the will of the Syrian people.
The Prince also met Defence Secretary Philip Hammond on his visit to London.
A Downing Street spokeswoman said: "Ahead of a meeting with the Defence Secretary, Philip Hammond, the Prime Minister met with Prince Salman bin Abdul Aziz of Saudi Arabia to discuss the wider bilateral relationship.
"They reaffirmed the importance of sustaining and developing the long term strategic bilateral relationship to ensure the security and prosperity of both the UK and Saudi Arabia.
"They discussed the situation in Syria, where they agreed that Assad had lost all legitimacy and that the efforts of the international community needed to be focused on ensuring a rapid transition that reflected the will of the Syrian people and on an immediate end of all violence and human suffering.
"On Iran, they agreed that the international community needed a concerted response to the threat posed by Iran's nuclear programme to the security of both the region and wider world.
"On the Middle East Peace Process, they agreed on the importance of negotiating peace between the Israelis and Palestinians as the best chance for securing security for Israel and justice for the Palestinians."