| Sine Wave Entertainment | |
| Sinewave Design | |
| Singer Instruments | |
| Singlescan Ltd | Leicestershire |
| SingleView | |
| Singula Bio Ltd | |
| Singular Intelligence | |
| Singular Photonics | |
| Sipcentric Ltd | West Midlands |
| Siraxis Ltd | |
| Sircles.net Computer Systems Ltd | Berkshire |
| SIRE Technology Ltd | Berkshire |
| Siren Search | West Midlands |
| Sirenum | |
| Siretta Ltd | Berkshire |
| Sirin Labs | Greater London |
| Sirius Insight | |
| SiriUS OpenSource | Berkshire |
| Sirius Partners | |
| Sirportly | |
| Sirrahsoft Ltd | |
| Sirv | Oxfordshire |
| Sisaf Ltd | |
| Sitala Bio Ltd | |
| Site Reach | |
| Site-Masters | Cornwall |
| Site-Street | Essex |
| SiteArt | |
| SiteBites | Berkshire |
| SiteBuilder Bespoke | Tyne and Wear |
| Sitebulb Ltd | |
| Sitecrafter | |
| SiteHelpDesk.com | Surrey |
| Sitehop Ltd | |
| Siteinspire | |
| Siteliner | |
| Sitely | Tyne and Wear |
| Sitemakers Ltd | Bristol |
| Siteon Ltd | |
| SiteVisibility | East Sussex |
| Sitewave | Hampshire |
| SiteWeavers Web Solutions Ltd | Hampshire |
| SiteWizard | Kent |
| Sitryx | |
| Six Degrees | Greater London |
| Six Search | |
| Six Two | Kent |
| Sixbysix | |
| Sixense | |
| Sixfold Bio | |