Fossil (Japanese: 化石の秘密 Mystery of the Fossils) is the name given to the third expansion of cards and Theme Decks for the Pokémon Trading Card Game in both English and Japanese.

• The English version of Fossil was originally planned to be released around January 2000 (based on original Jungle estimates), but because Jungle was pushed ahead, Fossil had to as well.

• Wizards of the Coast once again increased the set size by producing non-holographic copies of the holographic cards. They did, however, remove the Mew card that was in the Japanese set. The Mew would later reappear as Wizards Black Star Promos #8 (non-holographic) and #9 (holographic).

• Fossil completed the transition of the first 150 Pokémon (151 in Japan with the inclusion of Mew) to card form. Base Set, Jungle and Fossil totaled 158 Pokémon, meaning eight Pokémon appeared more than once. Pikachu and Electrode were in both Base Set and Jungle, while Gastly, Haunter, Magmar, Magneton, Raichu and Zapdos appeared in both Base Set and Fossil.

Of all English-released sets, Fossil contains the smallest number of cards.