Pokémon Colosseum is a Pokémon video game for the Nintendo GameCube console. In the main storyline the player is tracking down the creators of "Shadow Pokémon", Pokémon that have had their hearts turned to evil. You catch them and battle with them to make them good again. It incorporates elements of both the main hand-held games in the series as well as the Pokémon Stadium games. It is also notable for having a considerably darker storyline than most other Pokémon games.
The gameplay is very similar to the handheld games, but instead of moving in squares, your player moved freely in the 3D space. Battles are done in exactly the same way as the handheld series as well, but a few minor tweaks. Since there aren't any wild Pokémon in this game, the only way players can obtain Pokémon is by snagging Shadow Pokémon from opponents. Calling was a new combat option was added as well, which would return Hyper Pokémon from their agitated state back to normal, as well as awaken a party Pokémon who was asleep.
- Orre Colosseum
- Pyrite Town
- Realgam Tower
- Phenac City
- Outskirt Stand
- Shadow Pokémon Laboratory
- Agate Village
- Mt. Battle
- Snagem Hideout
- The Under
The game starts when an unknown building (Snagem Hideout) is shown in a canyon, with many people clad in uniforms walking around. An explosion causes everyone to run outside, where the main character (Wes, though you are able to name him anything you want) has stolen a device worn on the arm called a snag machine, which can catch Pokémon even if another trainer has already captured it. Wes rides off on a motor bike with his Umbreon and Espeon. He triggers a final explosion, destroying the building. After a quick stop at a rest station, he heads to Phenac City and finds two thugs with a bag. After beating the thugs in a battle and driving them off, Wes finds out that there is a girl inside (Rui, but she can also be named whatever you want). She can see Shadow Pokémon, and after a stop at the Mayor's house, the two begin a journey to stop people from creating shadow Pokémon.....
Wes starts at the Outskirt Stand, a dilapidated train in the middle of the desert that has been converted into a shop. With Espeon and Umbreon, Wes's first opponent is a Trainer named Willie. Wes then leaves the Stand and heads to the oasis-esque Phenac City in time to see two men dragging a sack. After defeating them in battle, Wes unties the sack to find Rui, a girl with the ability to discern Shadow Pokémon. They meet the mayor, Es Cade, who seems very bothered about the Cipher problem, but seems to do nothing about it. Later, upon leaving Phenac Colosseum, three Snagem grunts find Wes and Rui. The grunts then reveal to Rui that he was a member of Snagem, and an excellent Snagger. Wes then confronts one of the grunts, which results in the latter's defeat. Finding out that he is an expert at snagging Pokémon, Rui asks him to join forces with her in finding and snagging Shadow Pokémon.
As the game progresses, Wes becomes a target of a powerful organization known as Cipher. After leaving Phenac, he visits Pyrite Town, where Rui was kidnapped. In Pyrite, Shadow Pokémon are openly offered to winners of the town's Colosseum tournament - the local police force is powerless to stop the practice, so Duking (an influential man in Pyrite) asks Wes to enter the tournament and investigate. Wes enters the Colosseum challenge and defeats four trainers to win. Inside a nearby building, a Cipher Peon is about to present the Shadow Pokémon prize, but a friend recognizes Rui. The pair then battle their way through the building and a maze-like cave set in the rock behind. Eventually, they face and defeat Miror B., one of four Cipher Admins. Afterwards, Wes returns Plusle, a Pokémon Miror B. stole from Duking. A team of kids working in Duking's house introduce themselves as members of an anti-Cipher network, known as Kids Grid, who pledge their help to Wes and Rui.
Their next stop is Agate Village, dominated by a giant dead tree. Just as Wes is introduced to Rui's grandfather Eagun (the village elder), another villager runs into the room, telling them that the Relic Stone is under attack. Wes, following the elder to the center of the tree, fights off four Cipher agents before the Relic Stone is safe. After resting, Wes is given access to the Relic Stone, the only place where he can purify Shadow Pokémon. On a suggestion from the Elder, Wes decides to train his skills at the nearby Mt. Battle.
Upon reaching Mt. Battle, Wes and Rui realize that Cipher has already taken over the lowest level. Wes battles through nine peons before reaching the second Cipher Admin, Dakim. Dakim is under the possession of a Shadow Entei, which follows suit with Venus (Suicune) and Ein (Raikou).
After defeating Dakim, the player character heads to the Under, which is an underground "city" under the control of Cipher. More members of the Kids Grid are found here, as they tell the player of Venus, who has influence over The Under. After confronting Venus, and snagging the Shadow Suicune in her possession, she flees.
Later on in the game, the player characters head to the Shadow Pokémon Lab, where Pokémon are transformed into Shadow Pokémon. They ultimately face Ein, the head scientist behind the Shadow Pokémon project, and the final Cipher Admin.
Wes and Rui arrive at Realgam Tower. After facing the four admins once again, Wes obtains the key cards to reach the top of the tower. There, he is greeted by a large crowd, and a man named Nascour who then tells Wes that he will have to repay all of the debt that he had inflicted on them by facing four trainers for show. After Wes defeats all of the trainers and then Nascour, Nascour attempts to save face, only to be interrupted by Es Cade. Es Cade reveals himself to be Evice, the secret head of the Cipher organization, and battles Wes. When Wes defeats him, Evice attempts to escape by helicopter, but the legendary Pokémon Ho-Oh swoops in and blasts it out of the sky. Evice and Nascour are arrested.
Entei can be found at Mt. Battle, Suicune can be found at The Under TV Studio, and Raikou can be found at The Shadow Pokémon Lab, and Ho-Oh (Battle Mode) can be found at Mt. Battle. Jirachi is transferable to Ruby and Sapphire with the bonus pre-order disk.
Purification is a new topic added to this game, and is also in its sequel. Your character is equipped with a snag machine, which allows you to steal shadow Pokémon. with the Pokémon, it will have a plus next to the level. These Pokémon cannot level up until purified. To do this, you must battle with them a lot, until the meter where the Exp. Share should be is down all the way. Then, take them to the shrine at Agate Village. This will purify them. Also, the Time Flute can instantly restore one shadow Pokémon at the same village. The Time Flute summons Celebi, who instantly removes all traces of the strange shadow substance.
Pokémon Colosseum is compatible with all five of the main series Pokémon games. Items and Pokémon can be traded between them in the same manner that they can be traded between each other, however, the player must have defeated Evice in the main story mode in order to unlock it, have a Game Cube-Game Boy Advance cable, and a Game Boy Advance or GBA SP. All are required to do so.
- Pokémon Colosseum spawned a high-profile tournament in the United Kingdom entitled "Pokémon Colosseum Battlemaster 2004". The first round of battles was held at Toys "R" Us locations, with later battles taking place in movie theaters.
- The starter Pokémon, Umbreon and Espeon, already have some experience points at the start of the game.
- In Europe, the game was bundled with GameCube consoles shortly after its release.
- Despite being on the cover, Groudon and Kyogre are not available in the game unless traded from Ruby or Sapphire Versions, or Emerald Version. This is the only time that a Pokémon that appears on the game's cover is not found in the game, this happens also with Pokémon Battle Revolution. However, it should be noted that in Mt. Battle Singles Colosseum mode Kyogre is in the possession of trainer 100.
- This is the first Pokémon game to have Shadow Pokémon introduced.
- A manga adaptation of Colosseum's plot was printed in 2004 issues of the Japanese magazine CoroCoro Comic and titled Pokémon Colosseum Snatcher Leo (ポケモンコロシアムスナッチャーズレオ).
- Shiny Pokémon, when found, are likely to return to normal after being captured; this is due to the change in the trainer identification values after being captured. A Pokémon captured may turn into a shiny. A Shiny Pokémon in Colosseum will emit two orange 'orbs' when sent into battle, and alternate colors are visible as expected. The probability of a Pokémon being Shiny is the same as it is in the handheld games, 1/8192.
- Also, some Shiny Pokémon do not have the same coloration's as they do in the game, sometimes having slightly lightened or darkened colors or even having completely changed colorings altogether.
- The P-DA records the first Pokemon viewed ingame. Because of this, if the player has not yet encountered a species ingame, it is possible for the P-DA to record a shiny variant of the Pokemon in question for viewing in full 3D. This is the only way to view Shiny Pokemon in 3D for Gen III games. XD does not carry this feature over as it automatically records a normal variant of any Pokemon encountered.
- Many Generation I Pokemon are absent from this title. None of the Pokemon obtained from this game originate from Generation I (unless one counts the unreleased COLOS Pikachu).
- This title comes with a few Pokemon related errors. Many of these were fixed in Pokémon XD: Gale of Darkness:
- Jynx' arms are miscolored red, possibly relating to her old design.
- Frenzy Plant is labeled as Solid Plant, most likely a translation error. This text error however does not appear on a GBA screen during a Vs. Link battle (reading the correct Frenzy Plant instead).
- In the unused Mt. Battle prize room when the player receives a Pikachu, the text reads it is holding a LightBulb. The correct name, Light Ball, still displays everywhere else in Colosseum.
- An unused partially translated version of the Japanese Pokemon Colosseum Bonus Disc exists within the main game disc itself. Using Action Replay codes can re-enable its use. The menus displayed are akin to its Japanese counterpart (which include a Mt. Battle prize giveaway for Poke coupon rank holders and an Agate Shrine option to visit Eagun and encounter Celebi).
- Only some of the game text has been translated such as NPC dialogue. The European versions appear to have a few errors and glitches associated with this unused feature, possibly indicating it was heavily unfinished in PAL releases.
- The Mt. Battle prize location functions as it does in its Japanese counterpart. Any Pokemon game file containing a Silver Rank in Poke coupons is rewarded with a COLOS fateful encounter Pikachu holding a Light Ball (incorrectly translated as Lightbulb ingame.
- The Agate Shrine works just as it does its Japanese counterpart. Once all 48 Shadow Pokemon have been purified, the Colosseum player is able to receive an AGATE fateful encounter Celebi. Then Eagun will allow the option to transfer another Celebi into a GBA Pokemon core series game; once per save file up to 48 times.
- This game also contains the Japanese exclusive E-reader room, also partially translated. Using memory editing of the save file, one can re-enter the room and enable the unreleased E-reader features to obtain 3 extra Shadow Pokemon (Togepi, Scizor, Mareep). It is unknown if catching these 3 extra Shadow Pokemon will increase the total of the amount of unused AGATE Celebi distributions. 
- The E-Reader room can be seen during the demo reel of Pokemon Colosseum, viewable if one waits long enough at the title screen.
- The three extra Shadow Pokemon can be naturally obtained in XD: Mareep is snaggable, Togepi is obtained by an NPC and Scizor can technically be obtained in the form of Shadow Scyther.
There are four different backgrounds that will appear in order each time the title screen loads up. One can view them all by either allowing the demo reel movie to play (then exiting), or by entering and exiting the Main Menu screen.