The violin stolen from Frank Almond, concertmaster of the Milwaukee Symphony Orchestra.
Almond was walking to his car after a performance on Monday night when an assailant used a stun gun on him, causing him to drop the instrument.
The attacker then took the violin and fled to a waiting vehicle, which was being driven by a second suspect, according to Milwaukee police.
"It appears at this time that the violin was the primary target of this assault and robbery," police said in a statement.
Also Read
According to the symphony, Almond plays on a violin made by Antonio Stradivari from 1715, known as the "ex-Lipinski."
A police spokesman declined to comment on the instrument's exact cost, instead putting it in the "millions."
The Milwaukee police are working with the FBI's Art Crime Team, which specialises in high-end art thefts. The FBI team, in turn, works with Interpol to connect art dealers able to help locate stolen items throughout the world.
Before his death in 1737, Stradivari is thought to have made more than 1,100 instruments -- of which some 650 remain today -- according to the Smithsonian Institution.
He is the world's most celebrated violin maker.