The 1991 San Marino Grand Prix was a Formula One motor race held at Imola on 28 April 1991. It was the third race of the 1991 Formula One World Championship.
The 61-lap race was won by Brazilian Ayrton Senna, driving a McLaren-Honda, after he started from pole position. His Austrian teammate Gerhard Berger finished second, with Finn JJ Lehto third in a Dallara-Judd. It was also McLaren's first 1-2 finish since the 1989 Belgian Grand Prix. This was Senna's final win at the San Marino Grand Prix before his death in 1994.
Pre-race
With the team under new management having been sold by Cyril De Rouvre, Stefan Johansson was replaced at AGS by Formula One debutant Fabrizio Barbazza.
Qualifying
Pre-qualifying report
In the pre-qualifying session on Friday morning, Andrea de Cesaris was fastest in the Jordan, just under four tenths of a second faster than JJ Lehto's Dallara in second. De Cesaris' team-mate Bertrand Gachot was third, just a few hundredths behind Lehto. The fourth pre-qualifier was Eric van de Poele in the Lambo, who edged out the second Dallara of Emanuele Pirro. It was van de Poele's first progression through to the main qualifying sessions, and the first time in 1991 that a Dallara had failed to pre-qualify.
Apart from Pirro, those who failed to pre-qualify included Olivier Grouillard in the new British-built Fomet-1 chassis debuted by the Fondmetal team. Despite teething troubles which prevented him from progressing any further, Grouillard said he was happy with the new car.
| 1
| 10
|-
! 2
| 2
| Gerhard Berger
| McLaren-Honda
| 61
| + 1.675
| 5
| 6
|-
! 3
| 22
| JJ Lehto
| Dallara-Judd
| 60
| + 1 lap
| 16
| 4
|-
! 4
| 23
| Pierluigi Martini
| Minardi-Ferrari
| 59
| + 2 laps
| 9
| 3
|-
! 5
| 11
| Mika Häkkinen
| Lotus-Judd
| 58
| + 3 laps
| 25
| 2
|-
! 6
| 12
| Julian Bailey
| Lotus-Judd
| 58
| + 3 laps
| 26
| 1
|-
! 7
| 25
| Thierry Boutsen
| Ligier-Lamborghini
| 58
| + 3 laps
| 24
|
|-
! 8
| 8
| Mark Blundell
| Brabham-Yamaha
| 58
| + 3 laps
| 23
|
|-
! 9
| 35
| Eric van de Poele
| Lambo-Lamborghini
| 57
| Fuel pump
| 21
|
|-
! 10
| 26
| Érik Comas
| Ligier-Lamborghini
| 57
| + 4 laps
| 19
|
|-
! 11
| 7
| Martin Brundle
| Brabham-Yamaha
| 57
| + 4 laps
| 18
|
|-
! 12
| 15
| Maurício Gugelmin
| Leyton House-Ilmor
| 55
| Engine
| 15
|
|-
! 13
| 19
| Roberto Moreno
| Benetton-Ford
| 54
| Gearbox
| 13
|
|-
!
| 4
| Stefano Modena
| Tyrrell-Honda
| 41
| Transmission
| 6
|
|-
!
| 33
| Andrea de Cesaris
| Jordan-Ford
| 37
| Gearbox
| 11
|
|-
!
| 32
| Bertrand Gachot
| Jordan-Ford
| 37
| Suspension
| 12
|
|-
!
| 16
| Ivan Capelli
| Leyton House-Ilmor
| 24
| Spun off
| 22
|
|-
!
| 29
| Éric Bernard
| Lola-Ford
| 17
| Engine
| 17
|
|-
!
| 6
| Riccardo Patrese
| Williams-Renault
| 17
| Electrical misfire
| 2
|
|-
!
| 3
| Satoru Nakajima
| Tyrrell-Honda
| 15
| Transmission
| 10
|
|-
!
| 24
| Gianni Morbidelli
| Minardi-Ferrari
| 10
| Gearbox
| 8
|
|-
!
| 28
| Jean Alesi
| Ferrari
| 2
| Spun off
| 7
|
|-
!
| 30
| Aguri Suzuki
| Lola-Ford
| 2
| Spun off
| 20
|
|-
!
| 20
| Nelson Piquet
| Benetton-Ford
| 1
| Spun off
| 14
|
|-
!
| 5
| Nigel Mansell
| Williams-Renault
| 0
| Collision
| 4
|
|-
!
| 27
| Alain Prost
| Ferrari
| 0
| Spun off
| 3
|
|-
!
| 17
| Gabriele Tarquini
| AGS-Ford
|
|
|
|
|-
!
| 18
| Fabrizio Barbazza
| AGS-Ford
|
|
|
|
|-
!
| 10
| Alex Caffi
| Footwork-Porsche
|
|
|
|
|-
!
| 9
| Michele Alboreto
| Footwork-Porsche
|
|
|
|
|-
!
| 21
| Emanuele Pirro
| Dallara-Judd
|
|
|
|
|-
!
| 14
| Olivier Grouillard
| Fondmetal-Ford
|
|
|
|
|-
!
| 34
| Nicola Larini
| Lambo-Lamborghini
|
|
|
|
|-
!
| 31
| Pedro Chaves
| Coloni-Ford
|
|
|
|
|-
!colspan="8"|
|}
Championship standings after the race
;Drivers' Championship standings
{| class="wikitable" style="font-size: 95%;"
|-
!
!Pos
!Driver
!Points
|-
|align="left"| 10px
|align="center"| 1
| Ayrton Senna
|align="right"| 30
|-
|align="left"| 10px 3
|align="center"| 2
| Gerhard Berger
|align="right"| 10
|-
|align="left"| 10px 1
|align="center"| 3
| Alain Prost
|align="right"| 9
|-
|align="left"| 10px 1
|align="center"| 4
| Riccardo Patrese
|align="right"| 6
|-
|align="left"| 10px 1
|align="center"| 5
| Nelson Piquet
|align="right"| 6
|-
!colspan=4|Source:
|}
;Constructors' Championship standings
{|class="wikitable" style="font-size: 95%;"
|-
!
! Pos
! Constructor
! Points
|-
|align="left"| 10px
|align="center"| 1
| McLaren-Honda
|align="right"| 40
|-
|align="left"| 10px
|align="center"| 2
| Ferrari
|align="right"| 10
|-
|align="left"| 10px
|align="center"| 3
| Williams-Renault
|align="right"| 6
|-
|align="left"| 10px
|align="center"| 4
| Benetton-Ford
|align="right"| 6
|-
|align="left"| 10px
|align="center"| 5
| Tyrrell-Honda
|align="right"| 5
|-
!colspan=4|Source: