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, two adult daughters, and two half-siblings, Patti and Ron Jr., from Ronald’s second marriage to Nancy Reagan.