Quebec Superior Court Justice Guy Cournoyer convened court on Saturday to discuss the account after reporters covering the trial began following it over the noon hour.
Caught off guard, the judge said he was "speechless."
"One can imagine if, I don't know, untruthful, bizarre things could be attributed to me, I don't know," Cournoyer said.
"I'm not amused."
Cournoyer said he began receiving notifications around lunch that people were starting to follow a Twitter account existing under his personal email. Of them, the majority were journalists covering the trial who'd discovered it around the same time.
The judge weighed whether to disclose receiving the notifications, deciding ultimately to do so.
According to Twitter, the account was created in October 2012 and had no published tweets. The account followed two people — both law professors in the United States.
"It's very disconcerting because if anything is said during the day or the coming days attributed to me, it is not the case," Cournoyer said.
"I have no such account and I want that to be clear. I'm flabbergasted."
After his comments, all of the new followers, mostly journalists, disappeared.
Meanwhile, the eight women and four men deciding Magnotta's fate ended their fifth day of deliberations without reaching verdicts on the five charges against the accused.
He is charged with first-degree murder and four other offences in the slaying and dismemberment of Chinese engineering student Jun Lin in May 2012.