Gates says he will follow the initial investment up with another $50 million for start-ups working in Alzheimer's research.
"It's a miracle that people are living so much longer, but longer life expectancies alone are not enough", wrote Gates in a blog post.
He called it a growing problem, a huge problem and one where the scale of tragedy, even for those who remain alive, is extremely high.
The progress of the disease cannot be slowed with current treatments, which at best can only ease the symptoms.
Among the priorities identified by Gates is a better understanding of the disease process, earlier diagnosis, a more diverse drug pipeline, greater patient recruitment into trials and improved used of R&D data.
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Gates is optimistic that with focused and well-funded innovation treatments can be found, even if it may take a decade to reach them. Gates says he hopes that drugs to help Alzheimers sufferers will be developed within ten years, but said "it's possible that won't be achieved". Earlier this year biotech investment guru Neil Woodford pledged £15 million (around $19 million) to the fund, becoming its first backer beyond its core of Big Pharma founders (Biogen, GlaxoSmithKline, Johnson & Johnson, Eli Lilly, Pfizer, Takeda and Otsuka-subsidiary Astex), ARUK and the United Kingdom government.
The philanthropist, whose usual focus is on infectious diseases in poorer countries, said Alzheimer's caught his interest partly for personal reasons, and partly because it has so far proved such a tough nut to crack.
"Of all the disorders that plague us late in life, one stands out as a particularly big threat to society: Alzheimer's disease", Gates said.
"I know how terrible it is to watch people you love struggle as the disease robs them of their mental capacity, and there is nothing you can do about it", Gates wrote.
In 2015, an estimated 46.8 million people worldwide lived with dementia, a number set to double every 20 years reaching 74.7 million in 2030 and 131.5 million in 2050, according to World Alzheimer's Report 2015. This is something I know a lot about, because men in my family have suffered from Alzheimer's.