The Blues have agreed a three-year deal with the 18-year-old – who left Barcelona after just a year due to a lack of game time.
He penned a professional contract with Chelsea earlier this week and will now join the development squad ahead of the new season.
Mbuyama – likened to Virgil van Dijk – had a trial with Chelsea in the 2018/19 season but was unable to sign at the time due to the their transfer ban.
He instead spent the majority of his youth career with Dutch side MVV Maastricht.
But Mbuyamba then joined Barcelona last summer and played for their Under-19 side, making three Uefa Youth League appearances.
Xavier Mbuyamba has been unveiled as a Chelsea player and will wear the No3 jersey at Stamford Bridge.
Here he is with the new shirt being shown around Cobham.