The 24-year-old still had two years remaining on his previous deal having only joined the club in 2016, but Brentford were keen to tie him down to a longer contract and he has now committed his future to the Bees until 2021.
Yennaris was an ever-present in the Championship last season, scoring six goals in his 46 appearances to help Brentford to a top-10 finish.
"We are really pleased that Nico has agreed the contract extension. He has become an integral part of the team, as his appearances last season have shown," manager Dean Smith told the club's official website.
"He is continuing to improve in all aspects of his game and is becoming an accomplished goal scoring midfielder in the Championship."
Yennaris has scored eight goals in 93 appearances for Brentford since joining the club from Arsenal in 2014.