He will be wearing colourful vestments to take part in a solemn and historic occasion when he is enthroned as 105th Archbishop of Canterbury.
But before that, the Most Rev Justin Welby was dressed in a yellow lycra top and black leggings when he jogged past camera crews setting up in the precincts of the 900-year-old Canterbury Cathedral.
The 57-year-old - without his spectacles - surprised crews when he called out "hello" as he ran past at around 6.30am.
"He was very cheery - there was a little bit of cheekiness. He wanted to make sure we clocked him," said one member of a crew outside the cathedral.
A Lambeth Palace spokeswoman confirmed that Mr Welby had been for a jog - but declined to comment further.