Washo or Washoe (/ˈwɒ.ʃoʊ/; endonym wá꞉šiw ʔítlu) is an endangered Native American language isolate spoken by the Washo on the California–Nevada border in the drainages of the Truckee and Carson Rivers, especially around Lake Tahoe. While there were only 20 elderly native speakers of Washo as of 2011, since 1994 there had been a small immersion school that has produced a number of moderately fluent younger speakers. The immersion school has since closed its doors and the language program operates through the Cultural Resource Department for the Washoe Tribe. The language remains very endangered; however, there has been a renaissance in the language revitalization movement as many of the students who attended the original immersion school have become teachers.

Ethnographic Washo speakers belonged to the Great Basin culture area and they were the only non-Numic group of that area. The language has borrowed from the neighboring Uto-Aztecan, Maiduan and Miwokan languages and is connected to both the Great Basin and Northern California sprachbunds.

In 2012, Lakeview Commons Park in South Lake Tahoe was renamed in the Washo language. "The Washoe Tribe has presented the name Tahnu Leweh (pronounced approx. [tanu lewe]) which, in native language, means "all the people's place." It is a name the Tribe would like to gift to El Dorado County and South Lake Tahoe as a symbol of peace, prosperity and goodness."

Washo is usually considered a language isolate. That is, it shares no demonstrated link with any other language, including its three direct neighboring languages, Northern Paiute (a Numic language of Uto-Aztecan), Maidu (Maiduan), and Sierra Miwok (Utian). It is sometimes classified as a Hokan language, but this language family is not universally accepted among specialists, nor is Washo's connection to it.

The language was first described in A Grammar of the Washo Language by William H. Jacobsen, Jr., in a University of California, Berkeley, PhD dissertation and this remains the sole complete description of the language. There is no significant dialect variation. (Jacobsen's lifelong work with Washo is described at the University of Nevada Oral History Program.)

Washo shows very little geographic variation. Jacobsen (1986:108) wrote, "When there are two variants of a feature, generally one is found in a more northerly area and the other in a more southerly one, but the lines separating the two areas for the different features do not always coincide."

There are six distinct vowel qualities found in the Washo language, each of which occurs long and short. The sound quality of a vowel is dependent upon their length and the consonant they precede, as well as the stress put on the vowel.

Vowels marked with the acute accent ( ´ ) are pronounced with stress, such as in the Washo ćigábut (summer).