The centre-back, who has been in outstanding form this season, inked a four-and-a-half-year deal in January 2013 following a loan spell from Sheffield Wednesday.
And the 26-year-old former Motherwell man has now signed a one-year extension which commits him to the club for the next four seasons.
Dons manager Derek McInnes told official AFC website RedWeb: "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 said: "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."