It is illegal to marry your first cousin in Missouri. State law expressly prohibits marriage between first cousins, alongside other close relatives such as siblings, uncles and nieces, and aunts and nephews—the marriage would be considered void in Missouri. It is also illegal for any official to issue a marriage license to first cousins in the state.
However, marriages between more distant cousins—including first cousins once removed, second cousins, and beyond—are allowed in Missouri.
If a couple who are first cousins marry legally in another state where such unions are permitted, Missouri may recognize that marriage for certain legal purposes, but it is always illegal to perform or receive a marriage license for first cousins within the state.
Marriage between first cousins is not allowed in Missouri. Marriages between more distant relatives, such as second cousins, are legal.
Sources
[1] https://en.wikipedia.org/wiki/Cousin_marriage_law_in_the_United_States
[2] https://revisor.mo.gov/main/OneSection.aspx?section=451.020
[3] https://www.legalmatch.com/law-library/article/cousin-marriage-laws.html
[4] https://www.findlaw.com/state/missouri-law/marriage-laws-in-missouri.html
[5] https://dataminingdna.com/can-first-cousins-marry-in-missouri/
