The 27-year-old has signed a two-and-a-half year contract with the French club.
Diop has been in France before with Strasbourg, where he played for two seasons in 1999 and 2000, until his move to Xamax.
The central defender also had a brief loan spell at Norwich City, whilst with Strasbourg.
Lorient have also landed Nantes youngster Johann Cavalli, who has signed his first professional contract after a successful trial period.
