Assignor estoppel is the patent law doctrine that an assignor of a patent is estopped, or barred, from later contesting the validity of the patent. The logic behind this judge-made doctrine is that by assigning the patent, the patent-holder accepted its validity, and cannot contradict himself on that point later.
On January 8, 2021, the U.S. Supreme Court granted cert. in No. 20–440 to consider this question:
|“|| QUESTION PRESENTED:
In the Patent Act, Congress established that invalidity is a “defense in any action involving the validity or infringement of a patent.” 35 U.S.C. § 282(b) (emphasis added).
There is no textual exception to this command. The Federal Circuit nonetheless applies a judge-made “equitable” exception to the statute’s unqualified language known as “assignor estoppel.” Assignor estoppel prevents an inventor who has assigned a patent from later contesting the patent’s validity.
The question is whether a defendant in a patent infringement action who assigned the patent, or is in privity with an assignor of the patent, may have a defense of invalidity heard on the merits.