All adult-only all-inclusive Sandals resorts are located in Caribbean islands, and these countries require that visitors from the US have a passport in order to enter. So yes, you need a passport to go to a Sandals resort.
You can find Sandals resorts in the Bahamas, Jamaica, Curacao, Antigua, Barbados, St. Lucia, Grenada, and St. Vincent and the Grenadines.
Since these are all foreign countries, and there aren’t any Sandals resorts in the US, US citizens will need to bring their passports to visit any Sandals resort.

While some people think that because the Bahamas are so close to the US, they don’t require a passport, this isn’t the case. You will need a passport to enter the Bahamas.
It’s recommended that you have at least 6 months until your passport expires, as well, since some countries require that.
However, the good news is that you don’t need a special visa to visit these countries if you’re from the US.
If you’re looking for Caribbean destinations you can visit without a passport, these would be the US territories of Puerto Rico and the US Virgin Islands. But note that there aren’t any Sandals resorts in these islands.
If you don’t have a passport but want to travel to a Sandals resort soon, you can use the “Rush My Passport” service to get your application expedited.
Can you go to a Sandals resort without a passport?
If you are a US citizen, then no, you can’t go to a Sandals resort without a passport.

Can you travel to Jamaica without a US passport?
No, traveling to Jamaica requires a passport if you’re a US citizen.