A character class is a fundamental part of the identity and nature of characters in the Dungeons & Dragons role-playing game. A character's capabilities, strengths, and weaknesses are largely defined by their class; choosing a class is one of the first steps a player takes to create a Dungeons & Dragons player character. A character's class affects a character's available skills and abilities. A well-rounded party of characters requires a variety of abilities offered by the classes found within the game.

Dungeons & Dragons was the first game to introduce the usage of character classes to role-playing. Many other traditional role-playing games and massively multiplayer online role-playing games have since adopted the concept as well. Dungeons & Dragons classes have generally been defined in the Player's Handbook, one of the three core rulebooks; a variety of alternate classes have also been defined in supplemental sourcebooks.

These base classes have appeared as character classes in the core rulebooks of multiple editions of Dungeons & Dragons:

While the main character classes available have remained fairly consistent since the 1st Edition of Advanced Dungeons & Dragons, a variety of alternate base classes have been offered in supplemental books. The release of Unearthed Arcana in 1985, for instance, introduced the base class of Barbarian and reworked Paladins to be a type of the new base class "Cavalier". Oriental Adventures (1985) also introduced a number of alternate classes more appropriate for an Eastern setting. The 2nd Edition added several completely new base classes (e.g. Runecaster and Shaman); in addition, supplemental handbooks offered a variety of "kits" to customize each base class, and the Dungeon Master's Guide offered rules for creating new character classes. The 3rd Edition introduced five classes for use in creating non-player characters in its Dungeon Master's Guide. v3.5 Edition introduced the Artificer as a new standalone class for the Eberron campaign setting and it has continued to appear as an alternative class in the subsequent 4th and 5th Editions.

Non-core base classes are considered optional and do not always exist in all settings. For example, the Samurai class introduced in the Oriental Adventures book may not make sense in a game set in a standard European-style realm. Similarly, classes associated with psionics such as the Psychic Warrior do not apply to worlds without psionics.

Most editions of Dungeons & Dragons have allowed for the possibility to either advance in more than one class simultaneously, alternately taking levels in more than one class, or branching out in a second (or more) class at a specific point defined by the first class, a concept generally called "multiclassing".

In the 1st and 2nd editions, changing a character's class is difficult. Only those playing as humans can, and it requires extremely high stats to do so. This is called "dual-classing". Non-humans, on the other hand, can "multiclass" where they effectively learn two (and rarely even three) classes at the same time at the cost of slower level progression for that character.

The 3rd edition allows players to mix and match levels from any number of classes, though certain combinations are more effective than others. In addition, Prestige classes add more options for multiclassing. This edition offers the most freedom regarding multiclassing. There are, however, penalties to the rate of experience point gained if classes are added haphazardly. The 3rd edition version of Unearthed Arcana includes rules for gestalt characters which combine the advantages of two classes.