| Code | Name | Region |
|---|---|---|
| USXAF | Sioux Rapids | IA |
| USIRE | Siren | WI |
| USSS2 | Sisseton | SD |
| USISV | Sissonville | WV |
| US8IS | Sister Bay | WI |
| USYER | Sisters | OR |
| USIST | Sistersville | WV |
| USSIT | Sitka | AK |
| USSKJ | Sitkinak Island | AK |
| USSIE | Silver Springs | FL |
| USS4S | Silver Springs | NV |
| USSVR | Silver Springs | NY |
| USITK | Silver Star | MT |
| USSI3 | Silverado | CA |
| USS4D | Silverdale | PA |
| USSIF | Silverdale | WA |
| USS5V | Silverstreet | SC |
| USSO5 | Silverthorne | CO |
| USIER | Silverton | CO |
| USVET | Silverton | OH |
| USXVN | Silverton | OR |
| USILJ | Silverton | TX |
| USILV | Silvis | IL |
| USSIU | Simi Valley | CA |
| USSM8 | Simonton | TX |
| USSI9 | Simpson | NC |
| USUKJ | Simpson | PA |
| USSIJ | Simpsonville | KY |
| USSRY | Simpsonville | SC |
| USSIR | Siler City | NC |
| USSLG | Siloam Springs | AR |
| USXLP | Silsbee | TX |
| USS8I | Silt | CO |
| USZVA | Silva | MO |
| USSIB | Silver Bay | MN |
| US555 | Silver Beach | MA |
| USSEW | Silver Bow | MT |
| USSZI | Silver City | NC |
| USSVC | Silver City | NM |
| USSXV | Silver Creek | GA |
| USHQ6 | Silver Creek | MS |
| USZII | Silver Creek | NY |
| USVKY | Silver Grove | KY |
| USILK | Silver Lake | IN |
| US9KS | Silver Lake | NH |
| USIXL | Silver Lake | WI |
| USIVP | Silver Peak | NV |
| USJVP | Silver Plume | CO |
| USSIS | Silver Spring | MD |
| USS4N | Sidman | PA |