Poles "persecuted the Jews as vigorously" as German Nazis, according to a declassified US State Department document from 1946.
The new law makes it illegal to accuse Poles of complicity in the Holocaust and criminalises describing Nazi concentration camps in Poland as "Polish."
The legislation has been met with a furious backlash, drawing condemnation from Israel and the US.
Israeli Prime Minister Benjamin Netanyahu has described the law as an attempt to rewrite history and Holocaust denial.
Tensions between the two countries escalated after Polish Prime Minister Mateusz Morawiecki suggested that Jews were among the perpetrators of the Holocaust.
Netanyahu said that the comments reflected a "lack of understanding of history and lack of sensitivity to the tragedy of our people."
The US report on Poland's treatment of the Jewish people during World War II surfaced on Thursday (1 March).
It was distributed by the US Office of Intelligence Coordination and Liaison and was declassified in 1983 but had largely been forgotten until the Simon Wiesenthal Center obtained it.
The report states that anti-Semitism is deeply rooted in Poland's history and "was preached by political parties and church heads and practiced by officials high and low."
"By 1939, it was one of the distinguishing factors of the country's political, social and economic life," the document notes. "Anti-Semitic overtones" in Polish society "predisposed many Poles to the acceptance of Nazi racial theories," according to the report.
"There is evidence that Poles persecuted the Jews as vigorously as did the Germans during the occupation," it notes.
Rabbi Marvin Hier, the founder of the Simon Wiesenthal Center, said the report directly contradicts Polish leaders' arguments that anti-Semitism was caused by communism.
He said that the report contains evidence that shows Jews were treated as second-class citizens in Poland long before the communists came to power.
"In the jockeying for political preference in Poland after 1919, most of the major political parties – with the exception of leftist groups – followed an anti-Semitic line," the report notes.
"They have to acknowledge that anti-Semitism in Poland was a problem of longevity. You just have to read this report, which was not written by Jews, to see how real anti-Semitism was in Poland," Hier said.