Can I enter the U.S. as a tourist, marry my U.S. citizen partner, and stay to apply for a green card?

The marriage itself is not prohibited, but entering as a visitor while secretly intending to remain permanently can create a serious fraud or willful misrepresentation issue. A B-2 or ESTA traveler must be truthful about the purpose of the trip and eligibility for temporary admission. There is no automatic rule that every marriage followed by adjustment is fraud, but the government’s focus is what you intended and represented when you sought the visa or entry. This should be evaluated before travel, not after the wedding.