Short answer. Yes — that is precisely the situation Article 1363 of the Civil Code covers. Where you were mistaken and the other party knew or believed the document did not state your real agreement but concealed that from you, the instrument may be reformed to reflect what was actually agreed.
What the law says
When one party was mistaken and the other knew or believed that the instrument did not state their real agreement, but concealed that fact from the former, the instrument may be reformed.
Civil Code, Article 1363 — Reformation: Concealed Knowledge. Read the full provision →
What the provision actually requires
The text is compact: When one party was mistaken and the other knew or believed that the instrument did not state their real agreement, but concealed that fact from the former, the instrument may be reformed. Unpacked, four things must be present. There was a real agreement between you. The written document failed to express it. You were mistaken about that failure. And the other side either knew or believed the document was wrong and kept it to himself. The mistake need only be on one side. What supplies the unfairness is the other party's silence about a discrepancy he had already noticed and could have pointed out.
Reformation corrects the paper, not the bargain
This is a narrow remedy and it is easy to ask for the wrong thing. A court that reforms an instrument rewrites the document so that it says what the parties genuinely agreed. It does not annul the contract, it does not release you from it, and it will not hand you better terms than the ones you actually settled on. So it is no help if you simply regret the deal, misjudged the market, or now wish you had asked for more. The claim only works where there was a meeting of minds and the writing, by accident on your side and by exploited silence on his, departed from it.
You have to prove the real agreement, and the silence
Because a signed instrument is presumed to say what the parties meant, the burden on someone seeking reformation is heavy, and courts look for clear and convincing proof rather than a plausible story. Preserve the negotiating record: quotations, term sheets, earlier drafts and their tracked changes, the messages exchanged around the date of signing, and any minutes or notes. Evidence that the other side saw the discrepancy is what turns an ordinary drafting slip into this article's case — a comment in a draft, an internal note, a reply that dodges the point, or conduct afterwards showing he acted on the true terms while the paper said otherwise.
Where the remedy does not reach
Reformation is unavailable where no real prior agreement can be established, or where the parties intended the instrument exactly as it reads. The Civil Code also puts certain instruments outside the remedy altogether — among them simple unconditional donations and wills, and cases where the agreement itself is void. A party who continues performing under the written terms with full knowledge of the error may also find it harder to complain later. And the action does not remain open indefinitely, so raising the point in writing as soon as you discover it, and taking advice promptly, matters more here than in most contract disputes.