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 and Doohan get away, Doohan superior on the brakes at the end of the straight. Schwantz is a close 3rd.

Rainey drops Doohan, who is caught by Schwantz.

Pos.	Rider	Team	Manufacturer	Time/Retired	Points
1	Wayne Rainey	Marlboro Team Roberts	Yamaha	46:08.566	20
2	Kevin Schwantz	Lucky Strike Suzuki	Suzuki	+2.946	17
3	Mick Doohan	Rothmans Honda Team	Honda	+6.066	15
4	Wayne Gardner	Rothmans Honda Team	Honda	+23.218	13
5	Niall Mackenzie	Sonauto Yamaha Mobil 1	Yamaha	+33.205	11
6	John Kocinski	Marlboro Team Roberts	Yamaha	+37.614	10
7	Juan Garriga	Ducados Yamaha	Yamaha	+39.280	9
8	Didier de Radiguès	Lucky Strike Suzuki	Suzuki	+39.566	8
9	Doug Chandler	Roberts B Team	Yamaha	+52.121	7
10	Jean Philippe Ruggia	Sonauto Yamaha Mobil 1	Yamaha	+1:15.609	6
11	Eddie Laycock	Millar Racing	Yamaha	+1 Lap	5
12	Marco Papa	Cagiva Corse	Cagiva	+1 Lap	4
13	Cees Doorakkers	HEK-Baumachines	Honda	+1 Lap	3
14	Michael Rudroff	Rallye Sport	Honda	+1 Lap	2
15	Hans Becker	Team Romero Racing	Yamaha	+1 Lap	1
16	Josef Doppler	Doppler Racing	Yamaha	+1 Lap
17	Romolo Balbi		Honda	+1 Lap
18	Andreas Leuthe	Librenti Corse	Suzuki	+2 Laps
19	Helmut Schutz	Rallye Sport	Honda	+2 Laps
Ret	Sito Pons	Campsa Honda Team	Honda	Retirement
Ret	Michele Valdo	Team Paton	Paton	Retirement
Ret	Eddie Lawson	Cagiva Corse	Cagiva	Retirement
Ret	Adrien Morillas	Sonauto Yamaha Mobil 1	Yamaha	Retirement
DNQ	Nicholas Schmassman	Schmassman Technotron	Honda	Did not qualify
DNQ	Larry Moreno Vacondio		Honda	Did not qualify
Sources: [1][2]