The 2004 Japanese motorcycle Grand Prix was the twelfth round of the 2004 MotoGP Championship. It took place on the weekend of 17–19 September 2004 at the Twin Ring Motegi circuit.

This race was most notable for Makoto Tamada's home victory starting from pole, as well as Nakano's third place podium for Kawasaki and Barros' climb up to fourth from tenth on the grid.

One day before the race, on Saturday, Daijiro Kato's father presented two minibikes which will be sold this Fall in Japan. This was done to commemorate his son, who died last year during the 2003 Japanese Grand Prix at the now scrapped Suzuka Circuit after a high-speed collision with the barriers at the Casio Triangle.

After eleven rounds, Valentino Rossi is ahead of the rest with 209 points. Not far behind is Sete Gibernau in second with 180 points and third in the standings is Max Biaggi with 158 points.

Home hero Makoto Tamada took pole position on Saturday with a time of 1:46.673. Close second was John Hopkins and third was Valentino Rossi. The second row of the grid consisted out of Max Biaggi in fourth, Colin Edwards in fifth and Marco Melandri in sixth.

All riders take off and do their usual warm-up lap before lining up in their respective grid slots. As the lights go out, it is Hopkins who initially gets the better start and looks to lead going into Turn 1. However, it is Rossi manages to lunge ahead and leads going into the corner. As Hopkins turns in, he fails to spot Loris Capirossi who had a great start from seventh on the grid and collides with him as Capirossi loses the front and rear. This causes a chain reaction, bringing down Kenny Roberts Jr., Max Biaggi and Nicky Hayden with them. Six riders had now been eliminated on the opening lap, but Tamada – who had a bad start and thus lost multiple positions – was not one of the casualties. The marshalls run up to the stranded riders as the dust settles, revealing that all the riders except for Hayden had gone down (Hayden was stuck in the gravel). Biaggi is also stuck, the marshalls pushing his bike as a wounded Hopkins lies injured in the gravel. Capirossi has not moved since the incident and has been put on a stretcher, two of the marshalls lifting him up to bring him to the medical centre. Rossi meanwhile has been able to open up a gap to second place Tamada already. However, he quickly closes this entering Turn 11, pursuing Rossi for the remainder of the lap. Also at the straight before Turn 11, Sete Gibernau tries to pass Norifumi Abe by going up his inside, failing and having to stay behind him for the time being.

On lap two, the top six is as follows: Rossi, Tamada, Melandri, Shinya Nakano, Abe and Gibernau. Tamada has opened up a big gap to third place Melandri, who himself is coming under pressure from Nakano. At Turn 11, Tamada looks to be passing Rossi but stays behind for the time being.

Lap three and Tamada is still right behind Rossi, biting his time and not yet making a move. Behind him, Melandri is still being followed by Nakano, Abe, Gibernau and Carlos Checa.