The doctrine of assignor estoppel is a doctrine of United States patent law barring a patent's seller (assignor) from attacking the patent's validity in subsequent patent infringement litigation. The doctrine is based on the doctrine of legal estoppel, which prohibits a grantor (typically, of real property) from challenging the validity of his/her/its grant.

In Diamond Scientific Co. v. Ambico, Inc., the United States Court of Appeals for the Federal Circuit distinguished the policies applicable to assignor estoppel from those applicable to licensee estoppel. It therefore held that the doctrine of Lear, Inc. v. Adkins, which applies to licenses and holds that public policy requires that licensees not be muzzled from challenging the validity of possibly spurious patents, does not apply to assignments.

The UK counterpart to this doctrine is the doctrine of non-derogation from grants. Under this doctrine, as explained in British Leyland Motor Corp. v. Armstrong Patents Co., a seller of realty or goods is not permitted to take any action (such as bringing an infringement action) that will lessen the value to the buyer of the thing sold. Thus, the owner of copyright in the tailpipe of a motor car, having sold the motor car, may not then bring a copyright infringement action to prevent the aftermarket sale of replacement tailpipes to purchasers of those motor cars. The application of the UK doctrine outside real estate law has been limited subsequently to Leyland, however, to consumer protection contexts.