No trespassing signs can be useful to property owners—especially on large tracts of land or land that is not developed—to put persons entering the property on notice that the property is private property and if they do not have permission to enter, they may be committing civil and criminal offenses related to trespassing. In some states notice is required for a trespasser to have civil or criminal liability for entering the property without permission.

No trespassing signs may also protect a landowner against liability to a trespasser, as a lower duty of care is owed to trespassers in some states, and putting a person entering the property on notice that they are trespassing may strengthen the property owner’s defense against any liability claims if the trespasser is injured on the property.

In California, property owners can use 'no trespassing' signs to deter unauthorized entry and reduce liability. Under California Penal Code Section 602, trespassing is a criminal offense, and the presence of clearly visible 'no trespassing' signs can serve as notice that entry is not allowed without permission. This can establish that a person entering the property is doing so without consent, potentially leading to civil or criminal liability for trespass. Additionally, California law generally imposes a lower duty of care to trespassers compared to invitees or licensees. This means that if a trespasser is injured on the property, the owner may have a stronger defense against liability claims, especially if the trespasser ignored posted 'no trespassing' signs. However, this does not absolve property owners of all responsibility; they cannot willfully harm a trespasser and may still be liable for injuries caused by willful or malicious conduct.