Controlled by Hafiz Saeed, who has a $10 million US bounty on his head, the Milli Muslim League shot to prominence after fielding a candidate in a September 2017 by-election to fill a seat vacated by deposed Pakistani prime minister Nawaz Sharif.
Saeed has repeatedly denied involvement in the attacks.
"These designations seek to deny LeT the resources it needs to plan and carry out further terrorist attacks," the State Department said in a statement.
"Make no mistake: whatever LeT chooses to call itself, it remains a violent terrorist group. The United States supports all efforts to ensure that LeT does not have a political voice until it gives up violence as a tool of influence," the statement added.
Pakistan's Ministry of Foreign Affairs did not immediately respond to request for comment.
In October 2017, Pakistan's electoral commission barred the Milli Muslim League from contesting elections, saying the party had links with militant groups and could not be registered with the commission.
In March 2018 the Islamabad High Court ordered the election commission register the party.
Earlier, Saeed was placed under house arrest in January 2017 after years of living freely in Pakistan but a court ordered his release in November 2017.
Saeed's freedom in Pakistan, where he holds public rallies, has been a thorn in Islamabad's relations with India and the United States.