Obama's campaign said the candidate meant to say that his great-uncle, Charlie Payne, had helped liberate a part of the Buchenwald camp, not Auschwitz.
"Yesterday he mistakenly referred to Auschwitz instead of Buchenwald in telling of his personal experience of a soldier in his family who served heroically," said Obama campaign spokesman Bill Burton.
Burton said in the statement that Obama's great uncle served in the 89th Infantry Division that entered Germany in 1945 and on April 4 overran Ohrdruf, a sub-camp of the Buchenwald concentration camp.
Obama had made the Auschwitz reference in a Memorial Day speech on Monday.
More than 1 million people, most of them Jews, were killed at Auschwitz, an extermination camp in Poland. Buchenwald in Germany was mainly a forced labor camp, where some 56,000 people are believed to have died.
"I had an uncle who was ... part of the first American troops to go into Auschwitz and liberate the concentration camps," Obama said.
"And the story in our family was is that when he came home, he just went up into the attic and he didn't leave the house for six months," he said.
The Republican National Committee quickly pointed out that the Red Army had liberated Auschwitz in 1945, not American forces.