He's a libertarian. But he has also funded the secretive data-mining company Palantir, which works for the FBI and the CIA.
He co-founded PayPal and acted as its CEO before the company sold to eBay for $1.5 billion in 2002.
Since then he launched the global hedge-fund Clarium Capital and the Founders Fund, a venture capital firm. He was Facebook's first outside investor, making him a billionaire when the company went public.
He supports the Methuselah Foundation, whose goal is to reverse human aging, and the SeaSteading Institute, a libertarian group founded to create independent floating cities in international waters.
He's also launched the controversial "20 Under 20" initiative which gives 20 teens $100,000 to drop out of college for two years to pursue their own tech projects.
He is one of the most original, controversial thinkers in tech today.