The First Amendment to the United States Constitution provides that Congress shall make no law abridging (curtailing) the freedom of speech. Thus, free speech is only protected by the U.S. Constitution when it is the government that seeks to limit free speech. The First Amendment is inapplicable when a nongovernmental person or entity—such as a private business—seeks to limit free speech.

And some types of speech are afforded more protection than others. For example, commercial speech—speech that proposes a commercial transaction—is entitled to First Amendment protection, but less protection than political speech.

In the 1980 case Central Hudson Gas & Electric Corp. v. Public Service Commission, the U.S. Supreme Court developed a four-part test to determine whether commercial speech regulation violates the First Amendment:

1. Whether the commercial speech concerns a lawful activity and is not misleading;

2. Whether the government interest asserted to justify the regulation is "substantial";

3. Whether the regulation "directly advances" that government interest;

4. Whether the regulation is no more extensive than necessary to serve that interest.

In South Dakota, as in all states, the First Amendment to the United States Constitution protects freedom of speech from government infringement. This means that individuals in South Dakota have the right to express themselves without undue government restriction. However, this protection does not extend to private entities, such as businesses, which means that private businesses in South Dakota can set their own rules regarding speech on their premises. When it comes to commercial speech, which includes advertising or other speech proposing a commercial transaction, it is protected under the First Amendment but to a lesser degree than political speech. The Central Hudson Gas & Electric Corp. v. Public Service Commission case established a four-part test to assess the constitutionality of commercial speech regulations. South Dakota, like other states, would apply this test when considering the regulation of commercial speech to ensure that any restrictions are justified by a substantial government interest, directly advance that interest, and are not more extensive than necessary.