W is one of the least frequently used letters in the Spanish language. Although it is formally part of the Spanish alphabet, it is not very natural to the language. There are no original Spanish words that start with W, and only a few with foreign origin which have been adjusted to Spanish.
Foreign words starting with W used in Spanish
Native Spanish speakers consistently use many words from other languages in these days, especially words of English origin. Most of these foreign words are written and pronounced in the same way as in their original language. Examples include internet and tech related words:
wifi
website
webmaster
webcam
These can hardly be considered Spanish words though.
Words adjusted to Spanish that start with W
A few words starting with W became so familiar in the Spanish language and so commonly used that native Spanish speakers have adjusted their spelling and/or pronunciation to better fit the nature of the Spanish language. These words typically look and sound like a blend between the two languages and can be considered at least a little Spanish:
waterpolista (water polo player) with a typical Spanish ending
wikend (weekend) – alternatively, Spanish speakers also use the unadjusted form week-end
wellingtonia (giant sequoia) – this word has a natural Spanish equivalent, secoya gigante
whiski or wiski (whisky) and el whiskero (something or someone related to whisky)
Spanish verbs and adjectives that start with W
All the above words are nouns. One verb, also derived from whiski, is whisquear (to drink whisky).
You can hear other foreign origin verbs or adjectives that start with W, but they usually can't be considered part of the standard Spanish language.
Words that start with V in Spanish, but W in other languages
Some words of foreign origin that start with a W in their primary language changed the first letter to V in Spanish.
watt (the unit of power) is wat or vatio in Spanish
wagon begins with V instead of W in Spanish and has a long ó: vagón
There are not only words from English: waltz (the dance; from German) becomes vals in Spanish.
Summary: Spanish words that start with W
waterpolista
wikend
wellingtonia
whiski/wiski, whiskero, whisquear
wat
See similar lists of Spanish words that start with the letters A, I, K, X, and Y.