Johnson, who promised the British public he would “get Brexit done”, also posted a photograph of himself celebrating in front of the Union Jack flag.
The deal has been sealed just seven days before Britain exits the EU and one of the world’s biggest trade blocs.
“Deal is done,” a Downing Street source told Reuters. “We have taken back control of our money, borders, laws, trade and our fishing waters…
“We have delivered this great deal for the entire United Kingdom in record time, and under extremely challenging conditions … all of our key red lines about returning sovereignty have been achieved.”
European Commission President Ursula von der Leyen has said the deal was fair, balanced and right.
International Trade Secretary Liz Truss said separately the deal would lead to a “strong trading relationship” with Brussels and other partners around the world.
The deal had been all set to go ahead earlier on Thursday, but last minute haggling over fishing created a delay.
The UK formally left the EU on January 31 2020, but has since been in a transition period, during which rules on trade, travel and business would be discussed. So far, these things have remained unchanged, but the new Brexit deals will come into effect on January 1 2021.