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 Washington State, the use of 'No Trespassing' signs is recognized as a method for property owners to warn individuals against unauthorized entry onto their land. According to RCW 9A.52.080, it is a criminal offense to knowingly enter or remain unlawfully on someone else's property after being warned by signage. The signs must be posted in a conspicuous manner, designed to alert the public, and placed at reasonable intervals near the boundaries of the property. This serves to establish clear notice that entry is not permitted, which is a prerequisite for most trespassing charges. Additionally, under Washington law, property owners generally owe a lower duty of care to trespassers than to invitees or licensees. Posting 'No Trespassing' signs can help to reinforce a property owner's defense against liability should a trespasser get injured on the property, as it demonstrates that the trespasser was not invited and that the owner took reasonable steps to warn against trespassing.