Short answer. Generally no — under the parol evidence rule in Rule 130, Section 10, the writing is considered as containing all the terms agreed upon. But you may present evidence to modify, explain or add to it if, in a verified pleading, you put in issue one of four exceptions, including the writing's failure to express the parties' true intent.
What the law says
When the terms of an agreement have been reduced to writing, it is considered as containing all the terms agreed upon and there can be, as between the parties and their successors in interest, no evidence of such terms other than the contents of the written agreement.
Rule 130, Section 10 — Evidence of written agreements. Read the full provision →
The writing is presumed complete
This is the parol evidence rule. Once the terms of an agreement have been reduced to writing, the document is considered as containing all the terms agreed upon, and as between the parties and their successors in interest there can be no evidence of such terms other than the contents of the written agreement. The point is stability: parties who took the trouble to write their bargain down should not have it rewritten later by competing memories of what was supposedly said across the table. So your starting position is uphill — the court begins from the text, not from the negotiation.
The four exceptions that let evidence in
The rule is not absolute. A party may present evidence to modify, explain or add to the terms of the written agreement in four situations: an intrinsic ambiguity, mistake or imperfection in the written agreement; the failure of the written agreement to express the true intent and agreement of the parties; a challenge to the validity of the written agreement; or the existence of other terms agreed to by the parties or their successors in interest after the execution of the written agreement. A genuinely omitted term usually travels through the second exception — the writing fails to express what was truly agreed — or the first, as a mistake or imperfection in how it was drawn up.
You must raise it in a verified pleading
The exceptions have a procedural key: the party must put in issue in a verified pleading the ambiguity, mistake, failure of true intent, invalidity, or subsequent agreement being invoked. This is not a technicality to leave for trial. If your complaint or answer does not squarely allege, under oath, that the writing fails to reflect the real agreement, the testimony about the missing term can be shut out when you offer it. How the pleading is framed therefore decides whether your evidence is ever heard — which is a matter to settle with counsel before filing, not after.
The rule's own limits
Notice also who the rule binds: it operates as between the parties and their successors in interest, so a dispute involving a stranger to the contract stands differently. Terms agreed after the execution of the written agreement are outside the writing altogether and fall under the fourth exception — a later verbal modification is not erased by an earlier document. And the rule reaches beyond contracts: the section states that the term agreement includes wills. For your own case, gather everything that corroborates the omitted term — drafts, correspondence, receipts, conduct under the contract — because the exception must be proved, not merely alleged.