Short answer. Possession taken by someone who had no authority from you does not become yours automatically. Under Article 532 of the Civil Code, it only becomes legally yours once you ratify — that is, expressly or impliedly approve — what that person did in your name.
What the law says
the possession shall not be considered as acquired until the person in whose name the act of possession was executed has ratified the same
Civil Code, Article 532 — Who May Acquire Possession. Read the full provision →
Who may acquire possession and the rule for strangers
Article 532 of the Civil Code identifies three proper channels through which possession may be acquired on your behalf: you yourself, your legal representative, or your agent. A fourth possibility also exists — any person without any power whatever. This last category covers the situation where someone takes possession of your property without your knowledge or consent. The law does not automatically void what that person did, but it suspends the legal effect: the possession is not counted as yours unless and until you ratify it.
Ratification is the trigger
The word ratification is key. Once the person in whose name the act was executed ratifies that act, the possession is considered acquired — and, importantly, the ratification relates back to the moment the possession was taken. This means you do not lose the legal benefits of early possession simply because you were not the one who initially took it. But until ratification happens, the possession legally belongs to no one through you, and you have no rights that flow from it.
Negotiorum gestio and its separate consequences
Article 532 adds a reservation: ratification is without prejudice to the juridical consequences of negotiorum gestio in a proper case. Negotiorum gestio is the quasi-contract that arises when one person voluntarily manages the affairs of another without the latter's consent, typically in an emergency or when the owner is absent. If the circumstances qualify, the person who took possession on your behalf may have rights and obligations under that separate doctrine — including a right to reimbursement for useful expenses — regardless of whether you ultimately ratify the possession itself.
What this means practically
If you discover that someone has taken possession of your land or movable property without any authority from you, you have a choice. You can ratify what they did, which brings the possession under your legal belt from the time it was taken. Or you can decline to ratify, in which case you never legally possessed the property through that person's act. If the unauthorized possession involved expenses or created obligations, the negotiorum gestio rules may still impose duties on you even if you choose not to ratify. Consulting a lawyer will help you understand which path protects your interests best.