Constructive notice is a legal fiction used in the law of both common law and civil law systems to signify that a person or entity is legally presumed to have knowledge of something, even if they have no actual knowledge of it.

For example, one benefit of registering a trademark with the federal government of the United States is that the registration gives nationwide constructive notice that the trademark is owned by the registrant. Therefore, if another entity uses the mark, they will be treated as though they knew their use of it was an infringement, even if they had no actual knowledge of the registration, or the registrant's use of the mark.

Another common example of constructive notice is found in the law of civil procedure. Where a plaintiff files a lawsuit, but is unable to effect service of process on the defendant because the defendant is in hiding, or their whereabouts are unknown, most states permit the plaintiff to give constructive notice by either posting an announcement of the suit on property known to be owned by the defendant, or by publishing the notice in a local newspaper. Even if the defendant never sees the notice (or, at least, if it can not be proven that the defendant saw it), the court will go forward with the case as though the defendant was fully aware of the proceedings. In such a case, however, the defendant can later challenge the jurisdiction of the court to hear the case, at which time the plaintiff usually has to prove that he tried to effect service of process by other means, and was unable to do so.

Various forms of constructive notice have been challenged in the United States Supreme Court as violating due process. However, the Court has generally upheld such practices.