The Hutu ethnic group's revolution against Tutsi rule in the country forced Kigeli V into exile after a year in power.
The last king of Rwanda, Kigeli V, died on Sunday at the age of 80 in the United States, his website announced.
Born Jean-Baptiste Ndahindurwa in 1936, the monarch came to power in 1959. He ruled Rwanda for less than a year due to a revolution against ethnic-Tutsi rule in the country.
He and thousands of Tutsis were forced to leave the country when the Belgian colonial power favoured the Hutus in a revolt.
The revolution in 1961 led to the country's transition from a Belgian colony with a Tutsi monarchy to an independent Hutu-dominated republic.
Violence between the two groups lasted for three decades. Some 800,000 Rwandans were killed in the 1994 genocide against Tutsis.
The monarch lived in exile in several African countries before settling in Washington, DC, in 1992.
He set up the King Kigeli V Foundation in the US, a charity helping Rwandan refugees.
A 2013 profile in Washingtonian magazine found him in Oakton, Virginia, living off food stamps in subsidised housing.