The richest man in the US, Bill Gates, has issued the optimistic prediction that by 2035 there will be almost no poor countries in the world.

“In the next two decades, desperately poor countries will become the exception rather than the rule,” Gates wrote in an annual letter published on Tuesday by the Gates Foundation, a charity he and his wife, Melinda, established in 2000.

“Billions of people will have been lifted out of extreme poverty. The idea that this will happen within my lifetime is simply amazing to me.”

A few countries will be held back by war, politics or geography, he wrote, citing North Korea, barring a change in the country’s regime, and landlocked countries in Africa as exceptions. Inequality will be a problem; there still will be poor people in every region, he says.

But overall, Gates’ letter was optimistic. He backed up his predictions with photos, graphs and his own observations since turning his attention to philanthropy.

“Every nation in South America, Asia and Central America (with the possible exception of Haiti), and most in coastal Africa, will have joined the ranks of today’s middle-income nations,” the co-founder of Microsoft predicts.

“More than 70 per cent of countries will have a higher per-person income than China does today. Nearly 90 per cent will have a higher income than India does today.”

Poor countries are not doomed to stay poor, he insisted, noting that some of the so-called developing nations have already developed, and others are on their way.

Gates said he was optimistic enough about the future to predict that by 2035 there will be almost no poor countries based on the current definition of poverty.

Almost all countries will be “lower-middle income or richer” by that time, Gates wrote in the letter, which was distributed on Twitter and published at GatesFoundation.org.

“Countries will learn from their most productive neighbours and benefit from innovations like new vaccines, better seeds and the digital revolution,” Gates said.

“Their labour forces, buoyed by expanded education, will attract new investments.”

The Gates, whose net worth is estimated at $US72 billion ($A82 billion) by Forbes magazine, have published an annual letter for the last five years in an effort to set the global philanthropic agenda.

In addition to its predictions, this year’s letter focused on what Bill and Melinda Gates say are three myths that impede progress: that poor countries are doomed to stay poor, that foreign aid is a waste of money and that saving lives in developing countries just creates more starving mouths to feed.