| Code | Name | Region |
|---|---|---|
| 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 |
| USS23 | Sidney | IA |
| USS7I | Sidney | KY |
| USIJY | Sidney | MI |
| USSDY | Sidney | MT |
| USSNY | Sidney | NE |
| USSXY | Sidney | NY |
| USSDN | Sidney | OH |
| USIDM | Sidon | MS |
| USIEB | Sierra Blanca | TX |
| USSIM | Sierra Madre | CA |
| USAZS | Sierra Vista | AZ |
| USSI7 | Sierra Vista | CA |
| US83W | Sigel | IL |
| USSII | Signal Hill | CA |
| USS3M | Signal Mountain | TN |
| USQGU | Sigourney | IA |
| USSU9 | Sigurd | UT |
| USSIK | Sikeston | MO |
| USSHV | Shreveport | LA |
| USSHU | Shrewsbury | MA |