Washington designated the Pacific chorus frog (Pseudacris regilla) as the official state amphibian in 2007 (proposed by a third grade class at Boston Harbor Grade School in North Olympia, Washington). The Pacific chorus frog is a native amphibian found in every county of Washington state.

The Pacific chorus frog (also called Pacific tree frog) can be brown, tan, grey or green, and produce their charming sound by puffing up their throat sacs to three times the size of their heads. They are beneficial by eating insects including mosquitoes. Watch and listen to a video of a Pacific chorus frog from Michael Benard's website - Natural History of the Pacific tree frog.

Source:

State Symbols: Washington State Legislature

Links:

State Reptiles & Amphibians - 50 States List