The plural of phenomenon is phenomena.

The plural form of the word "phenomenon" is "phenomena". Forming plural nouns can be difficult. To form the plural form of the word, you'll base it on the last letter or last two letters of the singular word form. The word "phenomenon" is considered a common noun.

A phenomenon is an observable event or an exceptional occurrence, typically noteworthy in the fields of science and philosophy. The correct plural of phenomenon is phenomena. This follows the word’s Greek origin, where nouns ending in -on often have plural forms ending in -a.

Most dictionaries list phenomena as the plural of phenomenon, some include phenomenons, though notably this is the Americanized/modern plural form. ‍