The towering second-row forward, who has been capped twice by the All Blacks, joined the Exiles from native side the Hurricanes in September 2011.
London Irish director of rugby Brian Smith said: "Bryn Evans is a very important player for the club. He runs our lineout and is a key member of the leadership group.
"On top of that Bryn brings a lot of rugby intelligence to the team and he's a very popular and respected member of the group. It's fantastic he's committed to the London Irish cause. He is one of the true believers."
Evans has made 35 appearances for Irish, scoring three tries.