The childhood illness was eradicated in Spain almost 30 years ago thanks to routine vaccination programmes.
But it was widely reported in Spanish media that the child currently being treated at the Vall d'Hebron hospital in Barcelona had not been vaccinated.
The child first showed signs of the illness on May 25th when he presented flu-like symptoms including headache, sore throat and a fever.
Catalonia's Ministry of Health said that the child was being treated in the pediatric intensive care unit where his condition was described as “serious”.
Spain's ministry of health confirmed that the last recorded case of diphtheria dated from 1986.
Diphtheria is an infectious disease caused by the bacterium corynebacterium diphtheriae that usually affects young children and the elderly.
It is spread through the respiratory tract via coughing and sneezing and has a mortality rate of one in ten.