A club statement said Devlin had made the decision to move on as the Bees enter "an unprecedented period of change".
Devlin said: "I have nothing but pride when I look back on my seven years at Griffin Park and what the club has achieved in that time and how far it has come.
"However, you always reach a point when you feel there is another fresh challenge to be faced. I have reached that point and it seems like the ideal time for both myself and the club to make the change by the end of the year."
Brentford are in the process of building a new 20,000 seater stadium, which they are due to move into in 2020.
They also recently appointed a new head coach in Thomas Frank, who was promoted from assistant following the departure of Dean Smith for Aston Villa.