The versatile 24-year old, who scored 37 goals and made 17 assists in three-and-a-half years with Les Merlus, joins the Bees just two days before they are set to host Arsenal in their first-ever Premier League season.
Frank "keen" on former Lorient man
Speaking on the club website, manager Thomas Frank described the DR Congo international as, "someone who fits many different positions for us. He can play as a winger, a number ten, and as a striker.
"He has that pace and power which we think is really important to be a threat in behind. He has good ability to take on players and create overloads.
"He also has good pressing abilities; he will need to learn our style of play but he fits the pressing criteria we want from a winger, number ten or striker.
"He had one good season in Ligue 2 and one very good season in Ligue 1 and now we look forward to developing him further. I'm sure that he will help the team.
"He is a very good character as well; everyone knows that is very important for our culture that we only get good characters in. He is very keen to come in and help the team and we are very keen to get him in."
Giles: "He really wanted to join Brentford"
Co-director of football operations Phil Giles also spoke on the signing, and said that, "Yoane is a player we tried to sign last January to help us achieve promotion to the Premier League.
"Lorient preferred at that time to keep him to ensure they secured another season in Ligue 1, so it was a priority to make another attempt to sign him this summer.
"He really wanted to join Brentford so we are all very pleased that this transfer has finally come together after months of work."