The 23-year-old will join the Spanish side for a reported 50million euros (£41.87million) providing he passes a medical and agrees personal terms, which is not expected to be a problem given he has already posed for pictures in Barca clothing.
It is a blow for Chelsea, who were hopeful of bringing the centre-half to Stamford Bridge, but now must look for alternative defensive options.
A statement from Barca read: "FC Barcelona and Sevilla FC have reached an agreement for the transfer of Jules Olivier Kounde, awaiting the player passing a medical and the signing of contracts."
The deal continues Barca's spending spree this summer following the captures of Raphina from Leeds and Bayern Munich striker Robert Lewandowski.
Kounde joined Sevilla from Bordeaux in 2019 and made 133 appearances for the Andalucian club.
Chelsea lost defenders Andreas Christensen - to Barcelona - and Antonio Rudiger on free transfers this summer and although they have signed Kalidou Koulibaly, Thomas Tuchel is still on the lookout for additional arrivals.