Mr Key appeared on the popular TV chat show tonight, giving the regular and light-hearted top-10 segment a New Zealand flavour.
Among his offerings were that "New Zealand is a convenient 20-hour flight away" and "if you go in the next 30 days I'll pick you up at the airport personally".
He also told Letterman's audience of millions of viewers that "unlike the rest of the world, we like America".
Mr Key said he and Letterman joked about the distance between the US and New Zealand, with the talk-show host saying that New Zealand was his favourite country that he has never visited.
Letterman has a fear of flying, but said he took a great interest in New Zealand.
Knowing Letterman was a motor-racing fan, Mr Key presented him with a Indy 500 champion Scott Dixon T-shirt.
Prime Minister John Key had just over four minutes to persuade millions of Americans to visit New Zealand, and he had a fair go at it.
The show is watched by between three and four million viewers in the US, as well as having a worldwide audience.
Tourism New Zealand lobbied to get Mr Key on the show while he was in New York for the opening of the UN General Assembly, and chief executive George Hickton was at the scene.
"It's great to get that sort of profile," he told reporters.
"It's very hard to get noticed in this country and it's a very popular show...we just try to keep New Zealand's profile up as much as possible."
Mr Hickton was sure of success.
"We'll be noticed, and people will be talking about New Zealand. That was the whole aim of it."
Letterman set the scene after Mr Key walked into the studio with a cheery wave.
"How many years ago did you get on the plane to come here?" Letterman asked.
Mr Key admitted "I've aged a lot" and then had to confirm that mail actually did reach New Zealand. By pigeon, he said.
As the show ran through the top 10 Mr Key was relaxed and chatty with a host who sometimes gives his guests a hard time, although Mr Key admitted later he had been "a little bit nervous" before he went in.
Letterman questioned his producer at one point: "Why is he here, Dave?" and was told the prime minister's mission was to read out the Top 10.
The top 10 reasons for visiting New Zealand were:
1. Unlike most of the world, we still like Americans.
2. We drive on the left side of the road, like the British and Lindsay Lohan.
3. 70 percent of our energy is generated through renewable hydropower -- look, these don't all have to be jokes.
4. Visit in the next 30 days, I'll pick you up at the airport.
5. Get the whanau together, stay in a bach, crack open the chilly bin and slap on your jandals.
6. Down there Leno's on at 9 o'clock (a reference to Jay Leno's competing chat show).
7. It's like England without attitude.
8. It's only a convenient 20-hour flight away.
9. We have the loosest slot machines in the Pacific rim.
10. Auckland Airport now has a Cinnabon (a chain store specialising in sweet Danish pastries).
The show's writers had done their research, but they didn't get everything right. Auckland Airport doesn't have a Cinnabon and 60 percent of New Zealand's power comes from hydro.