| Name | Meaning | Gender | Origin |
|---|
| Rufus | Red | Male | Latin |
| Rugrat | | Unisex | Hebrew |
| Russell | Little red one | Male | French |
| Ryan | Little King | Male | Gaelic |
| Sasha | Defending men | Unisex | Greek |
| Sassy | Lively and spirited | Unisex | English |
| Savon | Soapmaker | Male | French |
| Shorty | Of less than average size | Unisex | English |
| Skeeter | | Unisex | Hebrew |
| Skipper | Master of a Ship | Male | American English |
| Skittles | | Unisex | Hebrew |
| Sky | Sky | Unisex | Norse |
| Smiley | | Unisex | Hebrew |
| Smudge | A blotch or smear | Unisex | English |
| Snickers | A brand of candy bar. | Unisex | English |
| Sniper | A skilled shooter who spots targets from a concealed place. | Unisex | English |
| Snookums | | Unisex | Hebrew |
| Snuggles | To curl up closely, cuddle | Unisex | English |
| Sonic | Approaching the speed of sound, fast-paced | Unisex | English |
| Spanky | | Unisex | Hebrew |
| Sparky | Animated, lively. | Unisex | English |
| Spaz | For a spazy kitty | Unisex | English |
| Speedy | Rapid motion, swift. | Unisex | English |
| Spirit | Part of the human being associated with the mind. Vigorous or animated state - spirited. Also distilled alcohol. | Unisex | English |
| Spunky | Spirited, plucky. | Unisex | English |
| Squeaker | | Unisex | Hebrew |
| Squeakers | | Unisex | Hebrew |
| Squeaky | | Unisex | Hebrew |
| Squirt | Slang for a small person | Unisex | English |
| Sterling | Little star - stellar | Male | English |