Pokemon Diamond and Pearl represent the fourth generation of the main Pokemon RPG series. The main plot remains consistent with the past games: the player is a fledgling Pokemon trainer who starts out by choosing one of three starter Pokemon. Those Pokemon are Turtwig (a grass type Pokemon that resembles a turtle), Chimchar (a fire type Pokemon that resembles a monkey), and Piplup (a water type Pokemon that resembles a penguin). After choosing, the trainer must travel the world, seeking out gyms in which they can battle and obtain badges. After all of the badges are obtained, they can battle the Elite Four and earn the title of Pokemon Champion.

New to these games, trainers will also be given a Poketch. This is a wristwatch-like device that appears on the bottom screen of the DS. It features more than 20 modes that are obtained throughout the game, each offering a different use. One mode may be just a simple clock, while another will tell the trainer how happy their Pokemon are.

Thanks to the internal clock in the Nintendo DS, a feature that was first introduced in the second generation of games is making a return. The game world will be perfectly synced to the DS's clock. If the game is played at night, it will be nighttime in the game world. All together there are five different time sections; morning, day, afternoon, evening, and night. The time of day affects several things within the game, like available wild Pokemon and berry growing.

As in past games, the main way of obtaining new Pokemon is by catching them in the wild. Diamond and Pearl feature several new additions to this mechanic. Firstly, all of the Game Boy Advance games from the main series (Ruby, Sapphire, Emerald, FireRed, and LeafGreen) can be inserted into the GBA slot of the DS. When inserted, different Pokemon will become obtainable in certain areas of the wild. Each game has a different set of Pokemon associated with it. There is also a new object, known as a Poketore, that helps track certain Pokemon. When in grass and using it, the grass will rustle where monsters are located. Based on how the grass moves, trainers can go after rare Pokemon right away, possibly finding an incredibly elusive shiny addition to their team. Rarer types can also swarm on certain areas of the map. Checking in with certain game characters can clue trainers in on where to go to catch those hard to find creatures. Also, some tree dwelling Pokemon can only be coaxed into the open by spreading honey on the tree and coming back later.

The battle system for the games is basically the same as all previous Pokemon games. Each Pokemon can learn a maximum of four moves. The desired move is selected at the beginning of a battle round. There is one fundamental change to the underlying system though. In past games, attacks were categorized as either special or physical. Which category they were depended on which elemental type they were. In Diamond and Pearl, any attack can be either category, and a new icon was put in place to differentiate them. Many old moves have changed categories now because of this.

The contests that were introduced in the Ruby/Sapphire/Emerald generation are also expanded in Diamond and Pearl. The visual judgment phase of contests require the trainer to dress up their Pokemon using the touch screen of the DS to place accessories. The better the accessories, the better score the Pokemon will receive for that round. Next, the Pokemon competes in a dance/singing competition. This mode plays like a rhythm game where the player taps four buttons on the screen. The final round of the contest remains true to the performance round in the old contests. Trainers choose the best sequence of moves to perform in order to achieve the highest possible score.

When the series transitioned between the Game Boy Color and Game Boy Advance hardware, old Pokemon could not be brought into the new games. That is not the case as the games move from the Game Boy Advance to the DS, but the Pokemon are not moved in a classic trading sense. Instead, there is a new area in the game known as Pal Park. When going to this area, if there is a Game Boy Advance version of the game in the GBA slot of the DS, the six Pokemon currently in that game's character's party will be set free in the park. The trainer will then be given a series of special Pokeballs that will capture a monster every time (like Master Balls). With those balls, they must search the park for the Pokemon that were set free. Rarer Pokemon (like legendary ones) will be harder to find in the park. The faster all six are found, the better the prize the trainer will receive, but regardless of time taken, they will always get to keep the Pokemon, effectively transferring them to the DS game. Even though the Pokemon are re-caught, they are still identical to the old one in every way. This event can only be performed for six per day. Because of this new transfer method, once a Pokemon has been moved from a Game Boy Advance game to a DS game, there is no way of moving it back.

For the first time in the series, Diamond and Pearl players can also go online to play with other people. There are several different modes that take advantage of the Nintendo WiFi Connection. Players can build a friend list by exchanging friend codes with one another. Once on each other's lists, they can trade and battle with each other. The game also features voice chat before, during, and after battles and trades. There is also a special underground dungeon area where players can go together. There, players can show off their own secret (decorated) base. They can also search for fossils, which can then be used to resurrect otherwise unobtainable Pokemon. While searching, they may also find gems that can be traded with merchants for decorations for their base. There is also a Capture the Flag game where players must enter their opponent’s base and retrieve a flag while avoiding obstacles and traps.

Besides using a friends list to connect with players, there is also a global WiFi feature. In this mode players can connect with other unknown players for battles and trades, as well as upload their records to a world-wide ranking system. There is also a global trade station where players can post trades that they want to make and peruse other players' desired trades.

Pokemon Diamond and Pearl will be available on April 22, 2007 in the US.