1991

Six Yamaha factory riders competed in the 1991 series. The new riders on the YZR500 this year were 1990 AMA Superbike Champion Doug Chandler and Adrien Morillas, who had performed well in the World Endurance Championship. Wayne Rainey from Marlboro Yamaha Team Roberts won six out of the 15 rounds of the series to take his second consecutive championship, and Yamaha took its seventh manufacturer's title in the class. Later on in the season, Yamaha made the YZR500 available to promising teams participating in the World GP and the All Japan series in efforts to revitalize the racing scene.

New regulations were in effect this season dictating that the rider's champion was to be decided by a rider's best points from 13 out of the 15 races. Rainey took six wins, four 2nd-place finishes, and three 3rd places before the 14th round in Le Mans, giving him a total of 233 effective points. With this, he was crowned the champion before the final round in Malaysia. Mick Doohan (Honda) came 2nd in the ranking.

In the opening round in Japan, Kevin Schwantz (Suzuki) was the winner, with Doohan 2nd, Rainey 3rd and John Kocinski 4th. Rainey won the following round in Australia to lead the ranking, and then backed it up by another win in the third round in the USA. However, after running in front of the pack in round four in Spain, he fell off the pace and only managed to come home in 3rd place due to excessive tire slippage. This allowed Doohan to take the lead in the championship ranking. This position stayed the same up until round eight.
Rainey took his third win in the eighth round in Jarama. At the next round in the Netherlands, however, ran wide and off into the dirt at a chicane while in the lead just 100 meters before the finish. This mistake gave Schwantz the opportunity to sneak past and take the win, leaving Rainey in 2nd place. Meanwhile, Doohan crashed out of the race, which put Rainey on top in the ranking. After this, he was also able to take his fourth win of the season in the tenth round in France. Rainey went on to take 2nd place in the 11th round Great Britain, followed by back-to-back wins in the 12th and 13th rounds in San Marino and Czechoslovakia. Round 14 in Le Mans saw a tight five-way battle between Schwantz, Doohan, Rainey, Kocinski, and Wayne Gardner, which eventually saw Schwantz come out on top. Rainey finished 3rd to clinch the championship title before the final race.

The final round was held in Malaysia, which hosted the Grand Prix for the first time. The fans were disappointed, however, when Rainey and Schwantz were unable to start the race due to injuries after crashing during the test runs. Kocinski started on pole and kept Gardner and Doohan at bay throughout the race to clinch his first-ever GP500 win.

Replacing the previous championship-winning YZR250 machine, Yamaha brought in the new TZ250M, which was based on the production racer. This bike was ridden by Alberto Puig (Spain) of the Ducados Yamaha team and Paolo Casoli (Italy) of Agostini Yamaha. Honda brought in their new factory machine for their main riders that enabled them to dominate the top five places in the ranking. Casoli finished his season in 10th.