MOSCOW — For the first time ever, ordinary Russians can now see documents that appear to confirm long-standing rumors that Vladimir Lenin had Jewish heritage.

In a country long plagued by anti-Semitism, such heritage can be a significant taint, especially for the founder of the Soviet Union who is still revered by many elderly Russians.

Among dozens of newly released documents on display at the State History Museum is a letter written by Lenin’s eldest sister, Anna Ulyanova, saying that their maternal grandfather was a Ukrainian Jew who converted to Christianity to escape the Pale of Settlement and gain access to higher education.

“He came from a poor Jewish family and was, according to his baptismal certificate, the son of Moses Blank, a native of (the western Ukrainian city of) Zhitomir,” Ulyanova wrote in a 1932 letter to Josef Stalin, who succeeded Lenin after his death in 1924....