Short answer. Yes. The Family Code requires marriage settlements, and any later modification of them, to be in writing, signed by the parties, and executed before the wedding takes place. An oral arrangement about property before marriage does not satisfy this requirement, whatever the couple may have agreed verbally.
What the law says
The marriage settlements and any modification thereof shall be in writing, signed by the parties and executed before the celebration of the marriage.
Family Code, Article 77 — Form of Marriage Settlements; Effect on Third Persons. Read the full provision →
The form the law requires
Article 77 sets three requirements for a marriage settlement together: it must be in writing, signed by the parties, and executed before the celebration of the marriage. All three apply not only to the original agreement but to any modification of it. A couple cannot rely on a verbal understanding about how their property will be treated once married, no matter how clearly they discussed it beforehand — the writing and signing are what the article treats as essential to the settlement itself.
Timing is part of the requirement
The settlement must be executed before the celebration of the marriage. This is not a formality that can be completed afterward; the article ties the settlement to the period before the wedding. A modification made later is likewise required to be in writing and signed, so the same formal standard follows any change the couple wants to make to what they originally agreed, not just the first document. That timing requirement matters for another reason too: under Article 81, everything stipulated in the settlement becomes void if the marriage never actually takes place, except for stipulations that do not themselves depend on the marriage being celebrated.
What registration adds, separately from validity
Article 77 also addresses third persons: the settlement and its modifications shall not prejudice third persons unless they are registered in the local civil registry where the marriage contract is recorded as well as in the proper registries of properties. This is a distinct point from the writing requirement between the spouses themselves — it concerns whether outsiders, such as someone dealing with the couple over their property, are bound by or can be affected by the settlement's terms.
What this means for couples drafting one
If you and your fiancé are agreeing on how property will be treated during the marriage, that agreement needs to exist as a signed written document completed before the wedding day, not as an understanding you intend to formalize later. If you later want to change its terms, the same writing and signature requirement applies to the change as well. Registering the settlement is a separate step from making it valid between you, but it matters if you want the terms to bind others who deal with your property.