yes you can, a person is aloud and has the right to marrry whomever they want whether based on religion,age(if over 18),or even gender(depending on location)
Of Course, a Russian can marry an American citizen!! All that you need to take note is that of the law of both the countries. If the marriage is to happen in the US, then the American citizen needs to sponsor the fiancee to the US on a F-1 visa. Once the visa gets approved and the fiancee is in the US, next step is getting married within 90 days of entering. Once the marriage is legalized, the American citizen can then sponsor a green card for the fiancee.