TMZ has reported that the Anchorman star's musician husband-to-be, Martyn LeNoble, has been sent death threats.
Police have launched an investigation into the messages which include: "I'll "blow your f**king head off!!!"
The 40-year-old's partner also received messages including: "You stole [Christina] away from me. If not for you she would be mine.
"I will not accept this. I am going to kill you!!!!!!!!!"
The anonymous threats went on to say: "I'm going to bust your head in for taking her away from me!!! Be afraid, be very afraid!!
"You think this is a joke? I want to see how funny you think I am when I blow your f**king head off!!!"
According to police, LeNoble didn't recognise the account user and the Twitter profile in question has now been deleted.
Applegate, who underwent a double mastectomy in 2008, said in an interview that she went through "a total emotional collapse" afterwards.
Referring to the line in her 2004 comedy Anchorman, she said that she misses her 'exquisite breasts sometimes'.
She's set to reprise her role in the 2013 sequel Anchorman: The Legend Continues. Watch a teaser trailer below: