Watson had resumed on Monday's fourth day of play in the third Ashes Test on 29 as Australia chased quick runs.
Caught on the boundary at 90 before the fieldsman Tim Bresnan tumbled over the rope, Watson quickly breezed into three figures for the fourth time in his 49-Test career.
He moved from a half-century to a hundred in 28 balls and his 103 came off 108 deliveries including 11 fours and five sixes.
Australia were able to declare on 6-369, setting England 504 to win.
"To have a license to take on the game certainly ... was a lot of fun for it to come off like it did today," Watson said.
Watson top-edged a catch to Ian Bell at cover on 103 and looked on like a wayward junior player, failing to run the single through to the bowler's end. Bell somehow dropped the catch but the bowler Bresnan threw down the stumps with Watson short of his ground.
"I wish Ian Bell had caught it, because that would have made it easier," Watson said.
"As a young kid you're taught to run through because there's always a chance of them dropping it.
"But I thought it was too much of an easy one. It wasn't good was it?
"I'm not going to enjoy watching that until the day I die.
"I'll make sure my boy Will (aged seven months) never sees that."