Cherie Blair, wife of British Prime Minister Tony Blair, joined Mrs. Bush and her daughter Jenna at a museum that offers an unsparing, haunting account of the 100-day slaughter in 1994 by Hutu militias that killed nearly half a million minority Tutsis and moderate Hutus.
The first ladies and Jenna Bush placed wreaths at one of the giant graves.
Graveside, the entourage bowed their heads in silence to coincide with observations in Britain marking one week since deadly bus and subway bombings in London.
Blair said it was especially poignant for her to be remembering the British dead at a site that marks those lost in Rwanda.
"I am very moved by what I have seen, also distressed that the world looked on while it happened," Blair said.
After touring the museum exhibit, the women, all somber, signed a visitors' book.
"This memorial serves as a tribute to the precious lives lost and a testimony to the courage of those who survived," Bush wrote. "The people of the United States stand with the people of Rwanda as they build a hopeful future."
There were no indications that Mrs. Bush planned to make a direct public link between what happened in Rwanda and the situation now in Darfur.
More than two years of conflict there have left tens of thousands dead and more than 2 million displaced in the Sudan, mostly as the result of a counterinsurgency by Arab, pro-government militias against black African rebels.