Most Revd Justin Welby had to miss his Christmas Day service because of his illness which was initially thought to be a bad cold.
He was ordered to take bed rest by doctors who had visited him at his home in Canterbury.
On Twitter he said:
Grateful for prayers & support from so many, & to such a gracious God. Better fast, brill @NHSEngland, a gift! Sorry for missing Christmas.
On Christmas Day the Archbishop was unable to attend read his own sermon and instead the Dean of Canterbury, the Very Revd Robert Willis, delivered a homily.
Pneumonia is swelling of the tissue in one or both of your lungs and is usually caused by an infection.
Symptoms include a cough, fever and difficulty breathing and in patients with other health conditions it can be very serious.
Lambeth Palace has stressed that Archbishop Justin is on the mend.