Convicted killer Michael Skakel could be headed back to prison after the Connecticut Supreme Court reinstated his 2002 conviction for the gruesome murder of his 15-year-old neighbor Martha Moxley.

Moxley was killed — bludgeoned with a golf club belonging to the Skakel family and stabbed in the neck with the broken-off handle — the night before Halloween in 1975 in the driveway of her Greenwich home. No physical evidence tied Skakel to the crime, but he was convicted because of incriminating things he said to friends and a weak alibi he gave investigators.

Skakel, the now 56-year-old nephew of Robert and Ethel Kennedy, served a decade of his 20-to-life sentence until he was released in 2013 when an appeals court ruled he didn’t get an adequate legal defense from his lawyer Mickey Sherman.

Connecticut’s highest court rejected that ruling yesterday in a divided 4-3 decision that found Sherman “rendered constitutionally adequate representation” — setting in motion a series of last-ditch efforts by Skakel’s appellate attorneys to keep him from heading back to the lock-up.