Ronald Reagan's Son Michael Reagan Dead at 80

One of the sons of former actor and President of the United States Ronald Reagan, Michael Reagan, has died at the age of 80 ... the Ronald Reagan Presidential Foundation and Institute confirms to TMZ.

Michael died on Sunday, January 4, Newsmax reports … his cause of death was not released.

Michael Reagan, who was adopted, was Ronald's oldest son … his mother was actress Jane Wyman -- Ronald's first wife.

Michael was an author of multiple books -- including "Lessons My Father Taught Me" and "Twice Adopted" -- a radio host and a lecture speaker on conservative politics.

Michael is survived by his wife, a son and a daughter, and two half-siblings, Patti and Ron Jr., from Ronald's second marriage to Nancy Reagan.