In a video message on the bid committee's website, the technology mogul and philanthropist heaped praise on the emirate and UAE.
"Dubai, as part of the UAE, has modernised itself, taken on tough projects, brought people together in a number of amazing ways and I think they would be a great location for a world expo," Mr Gates says.
He commends the UAE's policy to invest in future technology and improve learning.
"The UAE has taken its success and been willing to invest in new learning, new innovation," Mr Gates says.
"It's great to see that kind of long-term approach is part of their taking their success and becoming a hub of people from different regions.
"A lot of gathering is done here because they are a place that's looking forward. Everybody is willing to come and work together."
Expos play an important role in having people from across the world exchanging ideas and learning from one another, he says.
"Expos going back in history have let people see new innovations and let them think about 'how's that going to change life?'," Mr Gates says.
"They get people excited, get them to participate in different approaches, and so it brings people together, it gets us dreaming, and so it's a great institution."
Mr Gates also talks about his first expo, when he was only seven years old. "When I was a child I got the chance to go to the world expo in 1962 in my home town of Seattle.
"There was a lot of really great technology there, it was really quite inspirational."
The video shows footage of that expo, blended with present-day Dubai. The similarities are striking.
Seattle's monorail trains can be seen travelling above the city with the Space Needle Tower in the background. At the time, it was one of the tallest buildings in the world.
The scene quickly switches to the Dubai Metro track, with views of the Burj Khalifa in the background.
"The digital world is magical but I don't think it's going to replace meetings or expos anytime during my lifetime," Mr Gates says.