What is the highest purity of gold bar available in the market?

The highest purity of gold bar available in the market is 99.99%. This type of gold bar is also known as a "four nines" gold bar. Gold bars of this purity are typically produced by private mints rather than national mints. The Perth Mint in Australia is one example of a private mint that produces four nines gold bars. Gold bars of 99.99% purity are also sometimes referred to as "investment grade" gold bars.