Nick Skelton and Arko III had a clear round on the second circuit to become only the fourth British winners of the Rome Grand Prix since it began in 1926. The duo clocked 47.13 seconds to beat Michael Whitaker and Mozart des Hayettes by 0.49 seconds.
The 1996 German Olympic champion Ulrich Kirchhoff came third on Carino and former world champion Franke Sloothaak fourth on Legurio.
Malcolm Pyrah and April Love in 1975 were the last British winners.
The �30,000 reward takes Arko III's prize money in the last eight months to �665,000.