The 1991 San Marino motorcycle Grand Prix was the twelfth round of the 1991 Grand Prix motorcycle racing season.

It took place on the weekend of 16–18 August 1991 at the Mugello Circuit.

5th pole of the season for Kevin Schwantz.

Wayne Rainey gets the start from Wayne Gardner, Schwantz and Mick Doohan.

Rainey drops Doohan, who is caught by Schwantz.