In an op-ed piece published Friday in the Los Angeles Times, Knox wrote that Michelle Carter deserves sympathy and help, not a jail sentence.
Carter on Thursday was sentenced to 15 months in jail for the 2014 death of 18-year-old Conrad Roy III. Carter was then 17 and is now 20.
The 30-year-old Knox is no stranger to sensational trials drawing global media coverage.
The American exchange student from Seattle was convicted along with her Italian boyfriend in the 2007 killing of Knox's roommate, British student Meredith Kercher, in Perugia, Italy. Knox spent four years in jail but was exonerated by the Italian Supreme Court in 2015.