The former Motherwell and Sheffield Wednesday player put pen to paper on a four-and-a-half-year deal in January 2013, following a loan spell from the Owls.
And he has now signed a one-year extension which commits him to the club until the summer of 2018.
Dons manager Derek McInnes told the club's official website: "This announcement reinforces the strong message from the club. It wasn't necessarily an urgent piece of business for us but it is an important one in our plans for the future.
"It's good that Mark sees his long term future at this club and his desire to be part of a successful Aberdeen team is clear to see."
Reynolds, 26, added: "I had no qualms about signing an extension as there is a real feeling that the club is heading in the right direction and will achieve the success that the supporters crave."