The 27-year-old has been a key figure for the Bees, scoring 13 goals and providing 16 assists in 216 appearances across all competitions.
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In January, Janelt also became the first Brentford player to reach 150 Premier League appearances, highlighting his importance and consistency for the club.
“I’m delighted to get it done,” said Keith Andrews.
“Vitaly is a vital cog. He’s so important to the way we play, and he’s an influential figure in the dressing room and in the club.
“He’s been an immense player for us for numerous years, so I’m really pleased to secure Vitaly’s future. He’s going to be a big player for us for many years to come.”