The plural of cactus is "cacti." While the word "cactus" can be used as both a singular and plural noun, it is more commonly used as a singular noun. When referring to multiple cacti, it is more common to use the plural form, "cacti." However, the word "cactus" can still be used as a plural noun when necessary. For example, you might say, "I have two cactus plants in my garden," or "There are many different types of cacti." "Cactuses" is also an accepted plural form of cactus, but it's not as widely used as cacti.