Simon Collis, the UK envoy to Riyadh, was pictured dressed in the white robes worn by pilgrims, along with his wife Huda Mujarkech.
The image was tweeted by Fawziah Albakr from King Saud University. She wrote: “The first British ambassador to the Kingdom leads the pilgrimage after his conversion to Islam with his wife Mrs Huda in Mecca. Praise be to Allah.”
It is believed to be the first Hajj pilgrimage to Mecca ever made by a senior UK diplomat.
Collis, who is fluent in Arabic, thanked her. He added: “God bless you. In brief: I converted to Islam after 30 years of living in Muslim societies and before marrying Huda.”
The 60-year-old received warm praise and congratulatory messages on Twitter but has declined to give interviews about his faith.
The Hajj is an annual Islamic pilgrimage to Mecca and is one of the five pillars of the faith.
All able-bodied Muslims are required to perform it once in their lifetime.
It is seen as a chance to wipe clean past sins and start fresh.
According to Collis’s official Foreign and Commonwealth biography, Collis joined the FCO in 1978 and after studying Arabic, has served mainly in the Middle East and South Asia.
He has been British Ambassador to Iraq, Syria and Qatar. He has also served in Tunis, New Delhi, Amman and Bahrain.