The World Series of Poker's main event began last July with 6,844 players.
Eventually cut down a final nine, Eastgate beat 27-year-old Russian Ivan Demidov in the final in Las Vegas, Nevada.
Mr Demidov decided to go all in with his 16.45 million chips on two pairs (twos and fours).
Mr Eastgate, who had more than 120 million chips, won the hand with a straight (ace through five).
Speaking to American newspaper USA Today, he said: "I guess I haven't really realized how big this is.
"It will come in the next weeks or the next days. I will become very emotional at some point."
The winner added: "I was lucky he hit two pair... It's kind of inevitable when that happens he's going all in."
Mr Eastgate became interested in poker in high school, and decided to play online for money rather than going to college.
A mathematics fan in school, he said he was "fascinated" by the game and its "psychological aspects".
His previous biggest win was almost £30,000 at the Paddy Power 2007 Irish Open.
Mr Demidov won second-place prize money of more than £3.7 million.
He said: "I was just thinking he was bluffing. I read a few big bluffs at the wrong time."