Answer: Australia

Although currency has long been paper-based in the United States (and remains so), many countries around the world are slowly switching to polymer (plastic) banknotes. Why polymer? Although the diversity and sophistication of paper printing techniques are slowly catching up, there are more security features available in polymer currency as you can use traditional paper-based methods (like intaglio printing, security fibers, and such), but add in additional features that are difficult or impossible to recreate with paper (like transparent windows and watermarks).

The very first countries to experiment with polymer money were Costa Rica and Haiti starting in 1982. The process was strictly an experiment, however, and a very limited number of bills were circulated without widespread adoption. Over the next ten years, other nations dabbled with polymer money via small releases, commemorative banknotes, and the like, but none switched completely over to polymer money.

A decade after Costa Rica and Haiti took polymer banknotes for a spin, Australia released its first plastic currency in 1992. By 1996, they switched to producing all their banknotes on polymer. In the intervening years, the Australian government has continued to refine their polymer currency with increased security features and even increased usability; in early 2015, the Reserve Bank of Australia announced that the next generation of Australian currency would have tactile features to help visually impaired people distinguish between the different denominations.

Since Australia switched to polymer notes, additional countries have followed suit including:  Brunei, New Zealand, Papua New Guinea, Romania, Vietnam, Fiji, Mauritius, Canada, Israel, and Kuwait.