The letter K is among the less frequent letters in Spanish words. In general, words that begin with K in Spanish have their origin in other languages. Some of the words retain their exact spelling from the original language, while others have been adjusted to look more Spanish (typically by changing the ending or adding an accent).
Spanish nouns that start with K
A big group of K words commonly used in the Spanish language are metric units with the prefix kilo- (1,000 multiplier) and Spanish-sounding endings:
kilometro or kilometraje = kilometre
kilogramo = kilogram
kilovatio = kilowatt
kilólitro = kilolitre (there is an accent on o in kilo- here)
Other words that begin with K in Spanish are from more exotic regions and languages (the meaning in Spanish is the same as in English):
kárate (an accent on a is added in Spanish)
kiwi
koala
Replacing K with Q at start of Spanish words
Many words which begin with K in other languages start with Q in Spanish (Ke- is often transformed to Que-, or Ki- replaced with Qui-). With some words, both the Ke-/Ki- version and Que-/Qui- version are common:
kiosko or quiosco = kiosk
keroseno, kerosén, queroseno, or querosén = kerosene
Spanish adjectives that start with K
All the example listed above are nouns, as nouns are the most typical foreign origin words in Spanish. Adjectives can be derived from some of these nouns. For example:
kilométrico = kilometric, from kilometro (kilometre)
kaleidoscópico = kaleidoscopic, from kaleidoscopio (kaleidoscope)
kaki = khaki, from kaki (adjective which means having the khaki colour)
Spanish verbs that start with K
Similarly, you can derive Spanish verbs from some foreign origin nouns (usually by adding -ar or -ear ending):
kilometrar = to measure to kilometres
kidnapear = to kidnap
knockear = to knock-out somebody
Although none of these words feel 100% Spanish, they are the closest you can get to Spanish words that start with K.
See similar lists of Spanish words that start with the letters A, I, W, X, and Y.