The Knight First Amendment Institute at Columbia University called the practice "unconstitutional" and urged the freshman congresswoman to stop barring those with opposing viewpoints, a plea similar to one it made to President Trump.
The group called the congresswoman's personal @AOC account a "public forum."
"You use the account as an extension of your office to share information about congressional hearings, to explain policy proposals, to advocate legislation, and to solicit public comment about issues relating to government," the letter stated. "The @AOC account is important to you as a legislator, to your constituents, and to others who seek to understand and influence your legislative decisions and priorities."
As of Thursday, Ocasio-Cortez had 5.3 million followers on Twitter while her official congressional account, @RepAOC, boasted 188,000 followers. The group noted the congresswoman had the right to block users who post threatening speech.
In response, Ocasio-Cortez tweeted that fewer than 20 accounts were blocked for ongoing harassment and none were within her constituency.
"Harassment is not a viewpoint. Some accounts, like the Daily Caller, posted fake nude photos of me & abused my comments to spread it. No one is entitled to abuse," she posted.
Ocasio-Cortez did not respond to Fox News' request for comment.
Former New York Assemblyman Dov Hikind, a Democrat, sued Ocasio-Cortez in July for blocking him on her personal Twitter account.
"Alexandria Ocasio-Cortez has blocked me on Twitter yesterday ,apparently because my critique of her tweets and policies have been too stinging," Hikind said in a statement announcing the suit.
New York GOP congressional hopeful Joseph Saladino also sued Ocasio-Cortez after he was blocked.
The Knight Institute successfully sued President Trump over the same issue. He was forced to unblock several dozen followers last year after a federal appeals court upheld a July 2018 ruling that he couldn't block users on Twitter, even on his personal account.
Last week, Trump challenged the ruling, which said his blocking of people whose views he didn't like violated the U.S. Constitution.
Trump has 63.7 million followers and has used his account to make announcements that affect everything from public and foreign policy to Wall Street.