Someone posted about me on Facebook and publicly labeled me as her husband’s mistress without any evidence. What legal case can I file against her?

In the Philippines the Facebook post that calls you a “mistress” without any proof is actionable as defamation (libel) under Article 353 of the Civil Code and, because it was published online, also as cyber‑libel under Republic Act 10175 (Cybercrime Prevention Act). Both crimes are punishable by imprisonment (up to 12 years for cyber‑libel) and a fine, and the offended party may also file a civil action for damages (moral, exemplary and actual damages) under Articles 19, 20 and 21 of the Civil Code. To pursue the case you should (1) preserve the original Facebook screenshot (including URL, date‑time stamp and any comments), (2) obtain a Cease‑and‑Desist Letter demanding the removal of the post and an apology, (3) file a criminal complaint with the NBI or the PNP Cybercrime Division (the prescriptive period for libel is one year from publication), and (4) concurrently file a civil case in the appropriate Regional Trial Court for damages, attaching the same evidence plus proof of the harm to your reputation (e.g., loss of business, emotional distress).

If you need help drafting the cease‑and‑desist, preparing the criminal complaint packet, or assessing the damages for a civil suit, please let us know—we can guide you through each step and to protect you and seek redress.

Sincerely,
A-i-c

Hello:
Atty. Jofre here. Presuming this incident happened in the Philippines and the Facebook post was publicly accessible, what you described is not trivial gossip, it squarely falls under actionable legal wrongs.
Based on Philippine law, you have several solid remedies, both criminal and civil, depending on how far you want to push this.
Firstly, on criminal liability.
Labeling you publicly as someone’s “mistress” without proof is defamatory. Under Article 353 of the Revised Penal Code, this constitutes libel, and because it was done through Facebook, it becomes cyber libel under Republic Act No. 10175 or the Cybercrime Prevention Act. The elements are present if a.) the imputation is defamatory, b.) it was made publicly, c.) it refers to you, and d.) it was made with malice. Calling a woman a mistress is inherently damaging to reputation and moral standing, and malice is presumed when there is no factual basis.
This means you may file a criminal complaint for Cyber Libel before the Office of the Prosecutor, with possible penalties including imprisonment and fines. The prescriptive period for cyber libel is longer than ordinary libel, which works in your favor.
Secondly, on civil liability for damages.
Independently of the criminal case, you may file a civil case for damages under Articles 19, 20, 21, and 26 of the Civil Code. Article 26 is particularly important because it protects against acts that besmirch a person’s dignity, reputation, and peace of mind. Being publicly branded as a mistress affects your honor, emotional well being, and social standing. You may claim moral damages, exemplary damages if malice is shown, and attorney’s fees.
This civil action can be filed together with the criminal case or separately, depending on strategy.
Thirdly, on immediate remedies.
Before or alongside filing cases, we usually recommend sending a formal demand letter requiring a.) immediate takedown of the post, b.) a public apology, and c.) a written retraction. This serves two purposes. One, it gives her a chance to correct the wrong. Two, if she refuses, it strengthens the showing of malice and bad faith.
Fourthly, on evidence preservation.
This is critical. Take screenshots of the post, comments, reactions, profile of the poster, date and time stamps, and URL. Ideally, these should be notarized as a cybercrime affidavit or preserved through a lawyer assisted documentation. If the post goes viral or is shared, that further aggravates liability.
Now, a candid note for you. Courts look closely at whether there is any grain of truth, so the strength of your case depends on absolute falsity. If there is no relationship whatsoever, your position is strong. If there is any ambiguity, strategy becomes more nuanced, but it does not automatically defeat your claim.
I am the owner and Managing Partner of Recososa Law Firm, and we handle cyber libel, defamation, and online harassment cases nationwide. We have offices in Luzon, Visayas, and Mindanao. We can assist you from evidence assessment, drafting of the demand letter, up to filing and prosecution of the case. We can schedule an initial consultation via Google Meet or Zoom, or a physical consultation at any of our offices.
Sincerely,
ATTY. JOFRE B. RECOSOSA
Owner and Managing Partner
Recososa Law Firm