| Siren Biotechnology | California |
| Siren Digital Marketing | Minnesota |
| Sirenas | California |
| SirenOpt | |
| Siri InfoSolutions, Inc. | |
| Sirin Software | |
| SirionLabs | Utah |
| Siris Capital Group, LLC | |
| Sirius Automation | |
| Sirius Computer Solutions, Inc. | Texas |
| Sirius GT | Ohio |
| Sirius Networks | North Carolina |
| Sirius Therapeutics, Inc. | |
| Sirnaomics, Inc. | Maryland |
| Sirocco Energy, Inc. | California |
| Sirona Dx | |
| Sirono | California |
| Sirqul | Washington |
| SirsiDynix | Alabama |
| Sirtex Medical, Inc. | Massachusetts |
| Sirvisetti | New Jersey |
| SIS, Inc. | California |
| SIS, LLC | Georgia |
| SIS-TECH Solutions, L.P. | Texas |
| Sisco, Inc. | Michigan |
| Sisense, Inc. | New York |
| Siskiyou Communications, Inc. | California |
| Sisseton Wahpeton College | South Dakota |
| Sistema Universitario Ana G. Méndez | Puerto Rico |
| Sisterson & Co., LLP | |
| Sistine Solar | |
| Sistine Solar, Inc. | Massachusetts |
| Sisu Data, Inc. | California |
| Sisu Group, Inc. | Oklahoma |
| SISU, LLC | Texas |
| Sita Corporation | New Jersey |
| Sitation, LLC | North Carolina |
| Sitcur | California |
| Site Creations, Inc. | California |
| Site Designs by Lori | Pennsylvania |
| Site It now | |
| Site Launchers | |
| Site Mender, LLC | Texas |
| Site Revision, Inc. | Texas |
| Site Safe, LLC | Virginia |
| Site Savvy | |
| Site SEO | Florida |
| Site Smart Marketing, Inc. | |
| Site Strategies | |
| Site Symphony Design Studio | |