A desk mounted vertically adjustable stand up desk allows a conventional desk to be easily and quickly converted to a stand up desk for the user. The desk mounted vertically adjustable stand up desk includes a base plate adapted to be supported on a desk surface; a tower vertically extending from the base plate; a stand up desk surface member moveably mounted on the tower and configured to be vertically adjustable relative to the base plate; and a lock member coupled to the tower and configured to secure the stand up desk surface member in a desired user selected vertical position.