Since leaving the agency he has chaired the RICS Land and Society Commission, which in May published a report into how the property industry should operate under localism.
Beattie said that the commission’s work was now complete but that he would continue to take a close interest in how its recommendations would be achieved.
Commenting on his appointment, Beattie said: "I was brought up in rural West Sussex and have lived close to the South Downs for most of my life.
"The environment and heritage of the South Downs are uniquely precious, and it is a great privilege now to serve as chief executive of this new national park authority."
Beattie will start his new role on 17 January 2012.