A YDK file contains a Yu-Gi-Oh! deck used by various Yu-Gi-Oh! card dueling programs, such as YGOPRO, YGOPRODeck, Duelingbook, and Yu-Gi-Oh! Dueling. The YDK game data files are related to YGOPRO. What is File Extension YDK? YGOPRO is the primary software program that utilizes the Yu-Gi-Oh! Deck File file, originally developed by Open Source. The ydk file extension is associated with the YGOPro, an online dueling system made for playing Yu-Gi-Oh! duels available for various platforms. The YDK deck file contains Monster, Spell, and Trap cards that can be loaded into YGOPRO and used to duel other players.