Online court records say Salah Salahadyn entered the plea Monday after waiving his preliminary hearing.
The 41-year-old is charged with being a party to robbery. The charge carries a maximum penalty of 10 years in prison and a $50,000 fine.
Public defender Alejandro Lockwood declined to comment afterward.
Salahadyn and 36-year-old Universal K. Allah are both charged in the theft. A 300-year-old Stradivarius was stolen from a Milwaukee concertmaster in a stun-gun attack Jan. 27 but police recovered the instrument in good condition nine days later.
Salahadyn served five years for a previous conviction for art theft. Prosecutors say he'd been plotting for a while to steal a Stradivarius.