The 25-year-old Cape Verde international joins on a year's loan, with an option to buy.
The winger played against Paris Saint-Germain just last month, and featured in the Europa League, and Champions League qualifiers last season.
Mendes is a right winger, who adds to Forest's options in attack, which could take a dent with Michail Antonio on the verge of an exit.
The Mail reported yesterday that Forest have accepted a £7 million offer from West Ham for Antonio.
Mendes is an experienced option who could give Forest the lift they need, and is the second highly rated loanee from abroad to join in the last fortnight, following Dutch midfielder Kyle Ebecilio.
Forest remain under transfer embargo, permitted only to sign loan players and free agents.
Here is a look at how Forest fans reacted to the deal...
people saying Mendes is great are probably the ones who said Djebbour was the real deal #nffc— Pat O'Hara (@PatoharaPat) August 31, 2015
Ebicilio announcing himself. Lille announcing Mendes. What happened to a Fawaz tweet being the first news of a signing?! #nffc— Chris Hutchinson (@TheChrisH4) August 31, 2015
That's a serious singing that... Seen him play a few times and looks quality... #nffc— Jasper (@JasperRose96) August 31, 2015
Ryan Mendes , what a signing . Final part of the jigsaw . Cracking player . #nffc— @clashcityrocker (@nffc2009) August 31, 2015