However, he claims the existence of the world-saving technology is suppressed to allow business moguls to cash in on the oil and fossil fuel industries until they run out.
Dr Greer, a former emergency room doctor in the US, who gave it up to try to expose the truth about aliens, made the claims in an interview with uninterview.com.
He is a leading voice in the so-called disclosure movement in the US – conspiracy theorists who believe aliens are here on earth.
However, they say it is covered up and are fighting to get an alleged truth embargo lifted.
He said: “If along with the disclosure of intelligent life, you’re going to have the most disruptive technology in all of history come out… this becomes a big macro-economic issue and that is what the secrecy is partially rooted in.
“I began to realise that there were people inside government that know about these projects and there are people inside who do not know, and it has nothing to do with your rank, it doesn’t matter if you are the president.
“It doesn’t matter if you’re the CIA director. It only matters if the bureaucracy feels like you are going to go along with the agenda and keeping this secret.”