AI is most important tech advance in decades
he called it as fundamental as the creation of the microprocessor, the personal computer, the Internet, and the mobile phone.
"It will change the way people work, learn, travel, get health care, and communicate with each other," he said.
He was writing about the technology used by tools such as chatbot ChatGPT.
Developed by OpenAI, ChatGPT is an AI chatbot which is programmed to answer questions online using natural, human-like language.
The team behind it in January 2023 received a multibillion dollar investment from Microsoft - where Mr Gates still serves as an advisor.
But it is not the only AI-powered chatbot available, with Google recently introducing rival Bard.
I was one of the first people to get access to Bard and my colleagues and I are trying to put it through its paces.
So far it's given me a philosophical answer to the meaning of life.
It gave a competent potted history of Russia-China relations to a colleague covering the meeting between President Putin and Xi Jinping - unlike ChatGPT, Bard can access current affairs.
A programme editor asked it for a good running order for her news show. Start with the biggest story of the day, Bard suggested, and end with a musician or comedian. It also did a decent if generic job of a poem about trees and blossom.
I haven't yet started trying to get it to be rude to me, or about others. I'll report back on that…
Mr Gates said he had been meeting with OpenAI - the team behind the artificial intelligence that powers chatbot ChatGPT - since 2016.
In his blog, Mr Gates said he challenged the OpenAI team in 2022 to train an AI that can pass an Advanced Placement (AP) Biology exam - roughly equivalent to an A-level exam - with the strict rule that the AI could not be specifically trained to answer Biology questions.
A few months later they revealed the results - a near perfect score, he said, missing only one mark out of 50.
After the exam, Mr Gates said he asked the AI to write a response to a father with a sick child.
"It wrote a thoughtful answer that was probably better than most of us in the room would have given," he said.
"I knew I had just seen the most important advance in technology since the graphical user interface (GUI)."
A GUI is a visual display - allowing a person to interact with images and icons, rather than a display that shows only text and requires typed commands.
Its development led to the Windows and Mac OS operating systems in the 1980s, and remains a key part of computing.
And Mr Gates says he believes AI tech will lead to similar advancements.
Mr Gates, who co-chairs the charitable Bill & Melinda Gates Foundation, called on governments to work with industry to "limit the risks" of AI, but said the technology could be used to save lives.
"AI-driven improvements will be especially important for poor countries, where the vast majority of under-5 deaths happen," he wrote.
"Many people in those countries never get to see a doctor, and AIs will help the health workers they do see be more productive."
Some examples of this he gave include completing repetitive tasks such as insurance claims, paperwork, and note-taking.
But in order for this to happen, Mr Gates called on a targeted approach to AI technology in the future.
"Market forces won't naturally produce AI products and service that help the poorest," he said. "The opposite is more likely.
"With reliable funding and the right policies, governments and philanthropy can ensure that AIs are used to reduce inequity.
"Just as the world needs its brightest people focused on its biggest problems, we will need to focus the world's best AIs on its biggest problems."