"President Trump has freed so many children held in bondage to pimps all over this world," she wrote. "Hundreds each month."
She added: "He has broken up trafficking rings in high places everywhere. notice that. I disagree on some things, but give him benefit of doubt-4 now”.
The tweet is thought to refer to a conspiracy theory known as "The Storm", which emerged after the US President cryptically remarked that reporters were witnessing the "calm before the storm" during a photo opportunity with military leaders and their spouses before a White House dinner last October.
White House press secretary Sarah Huckabee Sanders refused to expand on what the president meant at the time, prompting online speculation about what he actually meant.
"The Storm" conspiracy has been traced to the online message board 4Chan, where it was propagated by an anonymous user identified as "Q" or "QAnon".
They alleged that high-profile Democrats and other famous people were members of child sex-trafficking ring. Mr Trump was said to be trying to break it up and arrest those involved in the operation.
Twitter users drew attention to the fact the 65-year-old actress and comedian has mentioned Q in past tweets as recently as November.
After a 21-year hiatus, she returned to American television last week in the much loved family sitcom Roseanne, drawing 18.4m viewers, which made it the highest-rated sitcom episode in more than three years.
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Mr Trump personally phoned her last week to congratulate her on the revival of the show, which focuses on the Conners, a working class Illinois family.
The reboot saw a family row erupt between Barr, a Trump supporter both on-screen and off-screen, and her sister Jackie who backs Hillary Clinton.
Her tweet about the theory prompted a mixture of confusion and anger among fellow users, with New York Times reporter Maggie Haberman simply replying: "What?"
Other public figures also questioned her, while John Dean, who worked in the White House under Richard Nixon, criticised the ABC television network, which broadcasts Roseanne for providing her with a platform.
Former MSNBC producer Michael Tomaso said Barr was heaping praise on a "fictional” president.
He tweeted: "This fictional president sounds amazing! Especially compared to the real one who’s been repeatedly retweeting white supremacist linked groups the last two years."
Responding to the criticism, Barr wrote: "I thought today was a good day to talk about freeing kids from sex slavery, since it is Passover. I didn't realise that so many were not aware of it. Anyway, no more opinions from me on twitter, it invites bullying. Moving on."
Barr, a one-time presidential candidate for the left-wing Peace and Freedom Party, appears to have moved to the right in recent years.
Her Twitter feed has been punctuated with articles from right wing news sites and she has adopted the rhetoric of the so-called “alt-right” movement on various occasions.
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