Elongated tetrahexahedral Au nanocrystals have been grown in high yields using a seed-mediated growth method. Morphological and structural characterizations show that these Au nanocrystals are single-crystalline and enclosed by 24 high-index {037} facets. They are more electrochemically active than octahedral Au nanocrystals that are enclosed by 8 low-index {111} facets. To date, there have been only a few reports of metal nanocrystals that are enclosed exclusively by high-index facets, including trisoctahedral Au nanocrystals enclosed by 24 {122} facets and tetrahexahedral Pt nanocrystals enclosed by 24 {037} facets. Our tetrahexahedral Au nanocrystals will be an important addition to the family of metal nanocrystals that are enclosed exclusively by high-index facets and will also be useful for fundamental catalytic studies on metal nanocrystals.