Correct spelling

supremacist

Correct spelling, explanation: the word supremacist is created out of the noun supremacy and a suffix -ist, which in this case signifies a political or ideological stance. In the process the y at the end turns into i, shaping the conjunction into the word supremacist.

Definition of supremacist:
adjective
1. A belief one type of people is inherently better/better suited for leadership than others (based on ethnicity, beliefs, etc.)
Hitler’s supremacist views about Jews led to horrible crimes against humanity.
noun
1. People who believe one type of people is inherently better/better suited for leadership than others (based on ethnicity, beliefs, etc.)
It’s good to be proud of one’s values and heritage, but we can’t support and condone the actions of supremacists.


Incorrect spelling

supremist

Incorrect spelling, explanation: it seems that supremist was a word used in the past, but it’s created on a different principle than supremacist, as the base word here is not supremacy, but supreme. The suffix -ist stays the same, but the meaning may be slightly lost, which is why it’s an incorrect form and supremacist is considered to be correct.