Greene ends GB's wait for a gold.
The 25-year-old ran a superbly-paced race to overhaul Javier Culson in the final few strides and add the world title to his European and Commonwealth crowns.
The Welshman's victory crowned a memorable evening for the British team, coming minutes after Hannah England produced a wonderful sprint finish to win a surprise silver in the 1500m.
Greene came round the final bend in third but was the strongest in the home straight, picking off first South African LJ van Zyl and then Culson as the Puerto Rican was tying up to win in 48.26 seconds, spreading his arms wide in celebration as he crossed the line.
Culson took the silver in 48.44secs with Van Zyl getting bronze in 48.80s.
The former Swansea youth-team footballer, who held his nerve after two faulty starts, said: "We've been waiting for this gold all week. I saw Hannah England's performance, saw her running round with the flag and I was like 'I'm going to get a piece of that action'.
"I was desperate to come away with the gold.
"If I'd had a bad season this year it wouldn't have set me up very well for next year.
"It's a lot easier during the winter months to train when you have a gold medal on the mantelpiece.
"Now I've got three from three from the last championships and I want to complete the set at the Olympics next year.
"I've had a lot of injures in 2006, 07 and 08 and was really struggling to make the squads and now look at me, I've totally changed that round.
"That stood me in good stead for where I am now. I've workedvery hard to get here and that's why I can deal with the expectations, because I remember being bottom of the pile." England's tears at missing out on selection for the last World Championships in Berlin two years ago were forgotten as she too produced a storming finish to take silver in the 1500m. The Oxford athlete looked to have d ropped out of contention with 200m to go, but showed plenty of strength down the home straight to finish 0.28 behind American winner Jennifer Barringer-Simpson. The 24-year-old Kelly Holmes protege, who came through the double Olympic champion's mentoring scheme 'On Camp with Kelly', said: "I was so calm in the last 200m. It was only coming down the home straight I realised the others were tying up, apart from Jenny of course. I was like 'what's going on'? "I knew I had a strong last100m."