The gold medalist was shown other weapons that authorities say were found in the car of Robert O'Ryan, who is accused of breaking onto a studio lot to meet Johnson as she competed on "Dancing With the Stars."
Johnson testified at the trial of O'Ryan, who has pleaded not guilty by reason of insanity to stalking and burglary charges.
Johnson, 18, was mostly composed throughout her testimony. She said she was nervous about appearing in court and repeatedly said she was afraid of O'Ryan.
In addition to the knife, prosecutor Wendy Segall showed the gymnast a handgun, shotgun and bulletproof vest allegedly found in O'Ryan's car.
Johnson testified for about 23 minutes.
O'Ryan, 36, of Florida did not visibly react as Johnson testified.
He has been jailed since last March, when he was arrested for investigation of jumping a fence at the studio lot.
Johnson said she considered resigning from the show, which she eventually won, after O'Ryan's arrest.
O'Ryan could face up to four years in prison if convicted. He also could face two years in jail if he is convicted of two misdemeanor concealed weapons violations.
