Onimusha 3: Demon Siege, released in Japan and Europe as Onimusha 3 (鬼武者3, Onimusha Surī), is a 2004 action-adventure game developed and published by Capcom. It is the third game of the Onimusha series and was released for the PlayStation 2 in April 2004, and later ported to Windows in December 2005. The story focuses on the returning hero, swordsman Hidemitsu Samanosuke Akechi, who is in his quest to defeat his nemesis, Nobunaga Oda, who wishes to conquer Japan with his army of demons known as Genma. Samanosuke, however, changes places with a French officer from the future named Jacques Blanc, and both have to adapt to their new worlds to get rid of Genma army and stop Oda Nobunaga's ambitions to conquer the world.

The game retains elements from previous games in the franchise including the use of multiple special weapons in order to fight enemies and absorb their energy to enhance the weaponry. This time the player primarily controls both Samanosuke and Jacques in separate scenarios. The two are able to exchange items in order to solve puzzles. In order to appeal to the Western market, the team set this game in Paris and chose French actor Jean Reno to portray Jacques.

Onimusha 3 has been generally well received by video game publications. Most of the writers' praise focused on the improved controls and the use of a 3D engine that allowed better combat and visuals. Critics also highly praised the CGI intro cutscene, which had martial arts legend Donnie Yen directing the fight sequences. One of the main criticisms was the game's short length. The game also generated good sales, with over 1.5 million copies sold as of May 2008, although according to Capcom it did not sell as much as they had hoped for.

Onimusha 3 is played in a real-time environment instead of pre-rendered backgrounds, although the camera is still controlled by the computer. Gameplay alternates between Samanosuke fighting in modern-day Paris and Jacques fighting in feudal Japan. Samanosuke fights using close range weapons, while Jacques uses an energy whip, which can also be used at certain points to swing across gaps. While Samanosuke cannot swing like Jacques he is able to use a bow in order to perform long-range attacks. Both characters also have defensive skills known as Issen that allows them to instantly kill an enemy by attacking just before the enemy makes contact with its attack.

Samanosuke and Jacques can gain elemental weapons, changing their fighting style and allowing access to magical attacks. Defeating enemies reaps souls, which when absorbed can restore health and magic, be used as currency for purchasing ammunition and health items, and allow Samanosuke or Jacques to enter a more powerful Onimusha state for a short time. If the player collects certain items during the game and completes it, Samanosuke will be able to use his weapons from the first game in the next playthrough. Armors with different properties can be obtained in the game but they require some items in order to work.

During certain portions of the story, players also get to control Michelle, who relies on firearms. Another minigame gives the players control over the lance fighter Heihachiro. Unlike Samanosuke and Jacques, Heihachiro and Michelle do not possess upgradeable weapons and cannot enter the Oni mode.

New to this game is a focus on time travel when solving puzzles. For example, if Samanosuke comes across a door in the present that has become too withered to open, Jacques will need to open the door in the past so that it will stay open for Samanosuke to progress. Things Samanosuke does in the present will not affect things in the past, but Ako is able to transfer some items between both timelines.

Having returned from mastering his newfound abilities as an Onimusha, Samanosuke Akechi returns to battle the Genma army in Japan 1582 as he fights his way to Honnō-ji Temple to finally put an end to Nobunaga Oda. Although Nobunaga was defeated by Jubei Yagyu years earlier, due to the fact that Jubei did not possess an Oni Gauntlet to seal Nobunaga away, this allowed the Genma to revive him. Though Samanosuke defeats Nobunaga's ward Mori Ranmaru, he is defeated by Nobunaga. Before the Genma Lord can kill him, he ends up being pulled through a portal that opened below him. Samanosuke awakens in Paris in the year 2004 where he aids a man named Jacques Blanc in fighting the Genma army that has begun its invasion near the Arc de Triomphe. The time portal reappears and teleports Jacques to feudal Japan.