Quiddler is a card-based word game designed by Marsha J. Falco and published in 1998 by Set Enterprises. Players compete by spelling English words from cards in hands of increasing size, each card worth various points. The game combines aspects of Scrabble and gin rummy. The word "Quiddler" is a trademark.

Before play begins, all the cards are shuffled. A game of Quiddler consists of eight rounds; the first round has a three-card hand, the second round has a four-card hand, and so on until the game ends with a ten-card hand. While there is a single-player variation, the regular game requires at least two players.

In the first round, the dealer deals out three cards to each player. The remaining cards form a draw pile. The top card is turned over to start a discard pile. The player to the dealer's left goes first. The player may choose the top card from either the face-down draw pile or the face-up discard pile. The player adds this card to their hand. The player ends their turn by discarding one card from their hand so that the player ends up with the same number of cards as were dealt. A player may not use a dictionary during their turn, but the other players may. Turns are taken in the same manner in a clockwise rotation among the players. The round continues until one player can go out.

A player can go out if they can use all the cards in their hand to spell one or more allowable words. After a player goes out, every other player then has one more turn. On the last turn, each remaining player uses as many cards as possible to spell one or more allowable words.

After the round is finished, points on the cards used to spell words are counted toward the player's score. If the player had unused cards, the points on those remaining cards are subtracted. Ten-point bonuses are added to the score of the player with the longest word and to the player with the most words.

The subsequent rounds follow in the same manner, with the hands increasing in size by one card each round. Thus, in the second round each player is dealt four cards, the third round five cards, and so forth. Points are added at the end of each round. The game continues until the end of the eighth round (ten cards in the hand). Depending on the number of players, the discard pile may need to be shuffled to resupply the draw pile.

The player with the most points at the end of the full eight rounds wins.

Players should decide on an English dictionary to be used during the game. An allowable word must appear as an entry in that dictionary or as one of the listed inflected forms of an entry word. Words must also use at least two cards. The makers of Quiddler have established several restrictions on the words used in a game: no proper nouns may be used; capitalized adjectives, such as "Iraqi" and "Scottish", may be used; prefixes and suffixes by themselves are not allowable words; words requiring a hyphen for proper spelling, such as "ex-wife" and "twenty-two" are also not allowed. The Quiddler rules disallow abbreviations, but do not differentiate between various forms. Players need to decide for themselves if they will allow acronym-derived words such as "laser" and "NATO". The rules contain no restrictions on English words with accented letters, such as "née", though the deck contains no such letters. Players must also decide for themselves if they will allow shortened word forms such as "ad" and "math".