Short answer. The person being sued, the party charged, must have signed, not the person suing. Article 1403 requires the agreement or a note or memorandum to be in writing, and subscribed by the party charged, or by his agent. Enforcement runs against whoever signed, so the plaintiff's own signature is not required.
What the law says
subscribed by the party charged, or by his agent
Civil Code, Article 1403 — Unenforceable Contracts and the Statute of Frauds. Read the full provision →
The signature that matters is the defendant's
Article 1403 says an agreement covered by the Statute of Frauds is unenforceable by action unless the same, or some note or memorandum, thereof, be in writing, and subscribed by the party charged, or by his agent. The phrase party charged means the person against whom the contract is being enforced in a lawsuit, the defendant. It is that person's signature, or his authorized agent's, that the law demands. The one bringing the suit does not need to have signed, because he is not the person resisting the obligation. The writing exists to hold the defendant to a promise he put his name to.
Why the party charged and not the plaintiff
The rule makes sense once you see what the Statute is for. It guards against false claims that someone agreed to an important contract when he did not. The danger is a plaintiff inventing an agreement the defendant never made. Requiring the defendant's own signature answers that danger directly: he cannot be forced to perform unless there is written proof, bearing his name or his agent's, that he actually assented. The plaintiff's signature proves nothing about the defendant's consent, so the law does not require it. This is why a one-sided writing, signed only by the party being sued, can still be enough.
What counts as a sufficient writing
The writing need not be a formal contract. Article 1403 accepts some note or memorandum of the agreement, so long as it is subscribed by the party charged. A letter, a signed receipt, or an exchange of notes can satisfy the Statute if it records the essential terms and carries the right signature. What the law is strict about is proof: evidence, therefore, of the agreement cannot be received without the writing. Without a qualifying writing, a court cannot even hear oral testimony to establish the covered agreement, unless the other side allows it or the contract has been ratified.
The contract is unenforceable, not void
It helps to know what non-compliance does and does not do. A contract that fails the Statute of Frauds is not void; it is merely unenforceable by court action. Article 1403 opens by saying these contracts are unenforceable unless they are ratified. The parties are free to perform it voluntarily, and if the party charged accepts benefits or fails to object in time, the defect can be cured. So the missing signature is a shield the defendant may raise, not an automatic nullity. Whether a particular note qualifies, and whether ratification occurred, turns on the specific facts and documents.