According to Sky Sports, Arsenal signed Guendouzi for a fee in the region of £8m, but the young man is relatively unknown in England.
So, what is there to know about the 19-year-old Arsenal signing?
Guendouzi was born on April 14, 1999, in the suburbs of Paris.
He was involved with football from a very young age, and joined the Paris Saint-Germain academy in 2005.
He spent nine years there before joining Lorient's youth side in 2014.
He played for the Lorient B team, making 24 appearances, before receiving his first team call up at 17-years-old.
He has shone in France's second tier.
Last season he made 18 league appearances for the side, as the club narrowly missed out on promotion.
For three months, he was frozen out of the side after a dispute with head coach Mickael Landreau.
He was reluctant to sign a new contract with the club, and he received interest from a number of teams, with Arsenal winning the race to sign the youngster.
It is unclear if the 19-year-old will go straight into Arsenal's first team, or go out on loan to gain more experience.
It looks like he could be going into a defensive or central midfielder position as a partner to fellow new signing Torreira.
Speaking about the arrival of Guedouzi, Arsenal head coach Emery said: "We are delighted Matteo is joining us. He is a talented young player and a lot of clubs were interested in him."
"He has big potential and gained good first-team experience last season with Lorient. He wants to learn and improve and will be an important part of our first-team squad."