The short answer
When you learn that your spouse is having an affair, the instinct is to start collecting evidence. Start instead by asking what you want from the case, because each goal rests on a different section, requires different proof and is brought against a different person. This guide covers separating the goals, lawfully obtained evidence and the deadline. For dividing marital property and children, see the divorce and marital property guide and the guide to maintenance and custody.
The law (Civil and Commercial Code). Section 1516 (1) allows a spouse to sue for divorce when the other spouse keeps or honours another person as a spouse, has a lover or is being unfaithful, or has habitual sexual relations with another person. The current text is gender-neutral following the marriage-equality amendment in force from January 2025. Section 1516 (2), serious misconduct that brings grave shame on the other spouse, is sometimes pleaded alongside. Section 1523 gives a right to compensation, and Section 1529 sets a one-year limit.
Our practical view. Two mistakes are common: letting more than a year pass, and gathering evidence in a way that is itself unlawful. Good evidence is evidence you obtained properly, with the date you learned the facts written down and a record of who saw what. The outcome still depends on the facts and the court's discretion, and we do not promise results.
Hypothetical. Nipa, in Khon Kaen, has been registered as married to Thanakorn for 9 years and they have one child. One day Malee posts a photo with Thanakorn on a public account saying they are a couple. Nipa takes a dated screenshot and comes to ask what to do next. We use this example throughout.
1. What are the possible goals, and who is the defendant?
| Goal | Legal basis | What must be proven | Defendant |
|---|---|---|---|
| (a) Divorce | Sec. 1516 (1) (and (2) if the facts fit); sue within Sec. 1529 | A registered marriage, and that the spouse keeps or honours another as a spouse, has a lover or has habitual sexual relations with another, and that you did not consent or condone (Sec. 1517) | The spouse |
| (b) Compensation from the spouse | Sec. 1523, first paragraph | The court grants a divorce on the ground in Sec. 1516 (1) | The spouse (joined to the divorce case) |
| (c) Compensation from the third party | Sec. 1523, first and second paragraphs | First paragraph: divorce on the ground in Sec. 1516 (1), and the person was kept or honoured or caused the divorce. Second paragraph: the person crossed the line with the spouse in the manner of a lover, or openly presented themselves as having such a relationship | The third party |
| (d) Stay married, claim from the third party only | Sec. 1523, second paragraph | The third party's conduct under the second paragraph, for example crossing the line in the manner of a lover or openly presenting a romantic relationship | The third party |
An important note: the text of the second paragraph of Section 1523 does not say a divorce judgment is needed first, so it is the route for a spouse who wants to keep the marriage. We should not say you will win. The court will look at whether the conduct fits the words "crossing the line in the manner of a lover" or "openly presenting", and the amount of compensation is in its discretion.
In our example Nipa can pursue goal (d) if she wants to stay. If she decides to divorce, goals (a), (b) and (c) can be brought together in one case.
2. Must the marriage be registered?
Sections 1516 and 1523 speak of a spouse, so the marriage must be registered. Couples who live together without registering have no right to sue for divorce and no right to compensation under these two sections. If you are not sure your marriage was validly registered, first obtain a marriage certificate from the district office.
The ground in Section 1516 (1) is gender-neutral after the marriage-equality amendment, so it applies equally to every registered spouse.
3. Which evidence is safe, and which is risky?
The principle is to rely on what you can reach lawfully in your own right. This table is general guidance and does not guarantee that a court will accept every item.
| Type of evidence | Risk | What to do |
|---|---|---|
| Public social-media posts (anyone can view) | Low, if you only capture and keep them | Take a screenshot showing the URL, date, time and account name, and keep the link |
| Photos taken in public places | Low to moderate | Take only what is needed to show the event, do not trail or harass, and do not post them |
| Messages you received directly, for example from the third party | Low | Keep the whole conversation with its date data, unedited |
| Witnesses who saw it themselves | Low | Note names, the date they saw it and what they saw, in their own words |
| Hotel or rental documents | Do not go and get or buy the data yourself | Ask your lawyer whether the court can order the documents once a case is filed |
| Private investigator reports | Moderate to high, depending on the methods | Ask first about methods: no trespass into private places, no hacking, and the investigator must be willing to testify |
Never do these:
- Log in to your spouse's phone, LINE, Facebook or email without permission, even if you know the password
- Install a tracking app or a message-intercepting app on their device
- Hide a camera or recorder in a place where they are entitled to privacy, such as a bedroom that is not yours or a rented room
- Open mail or parcels addressed to someone else
- Post accusations about your spouse or the third party in public
4. Why unlawful gathering can backfire
- Computer Crime Act B.E. 2550, sections 5 to 7. Section 5 concerns unlawful access to a computer system protected by a specific access measure. Section 6 concerns unlawfully disclosing such a measure, for example a password, where damage is likely. Section 7 concerns unlawful access to computer data protected by a specific access measure. Each carries imprisonment or a fine or both, at different levels.
- Personal Data Protection Act B.E. 2562. There is an exemption for collecting and using data purely for personal or family purposes, but circulating other people's data or passing it on may go beyond that exemption, so take advice before sharing anything with anyone.
- Criminal Code sections 326 and 328. Imputing something to another person to a third party in a way likely to damage their reputation is defamation, and publishing by document or online post carries a heavier penalty under Section 328. Even if you believe it is true, you must still consider the defences in Sections 329 and 330. See the online defamation guide.
In civil cases the rules on admitting improperly obtained evidence are not simple, and we do not say that a court will or will not admit it in every case. What we can say is that you may face criminal allegations yourself, the other side will use it to attack your credibility, and your case may lose time on the point. The same approach applies to tracing assets and money: see checking money trails before divorce lawfully.
5. How to preserve evidence so it can be used
Evidence whose source or date is unknown is hard to use. Write down when you got it, where it came from and who saw it, and keep the originals together. Do not edit files.
Evidence log (one row per item)
No.: 001
Date you first learned of this: ___/___/___ (matters for the 1-year limit)
Date and time of the event/post: ___________
Type: public post / photo / message received / witness / document
Source (URL or place): ______________________
Who saw or recorded it, and when: ______________________
Original file kept at: ______________________
Relates to goal: divorce / compensation from spouse / compensation from third party
Notes (for example, account may be deleted, document request needed): ______________________
6. The deadline to check
Section 1529 provides that the right to sue on the grounds in Section 1516 (1), (2), (3) or (6), and the right to compensation under Section 1523, lapses one year after the claimant knew or should have known the facts they can rely on. The start date is therefore not always the date the act happened, but do not guess it. Write down the date you learned and leave time for a lawyer to check before the year ends.
A caution about knowing and staying: Section 1517 bars a spouse who consented to or condoned the act from relying on the grounds in Section 1516 (1) and (2), and the third paragraph of Section 1523 bars compensation in that case. Reconciling or living together after you find out should be discussed before you decide.
In our example Nipa saw the post on 3 October. She should record that date and book a lawyer within a few weeks, not wait until the year is nearly up.
7. Other points to consider alongside
- Property and children are separate from the affair. See the divorce and marital property guide and the guide to maintenance and custody. For an existing marriage agreement, see what to agree in a prenuptial agreement.
- If there is violence, safety comes before evidence. Leave, go somewhere safe, report to the police or call 191, and keep photographs of injuries or a medical certificate. Section 1516 (3) also covers serious physical or mental abuse.
- Divorce by mutual consent. If both sides agree, you may register the divorce under Sections 1514 and 1515, which need a written agreement, at least two witnesses and registration. Put the terms on property, children and compensation in a written agreement, because compensation under Section 1523 is tied to a divorce judgment, so a consent divorce should settle that point in writing from the start.
8. Checklist before seeing a lawyer
Documents and identity
- Copy of the marriage certificate or a certificate of marriage
- Copies of your and your spouse's ID cards and house registration
- Name and address of the third party, as far as you know them lawfully
Lawfully obtained evidence
- Screenshots of public posts with URL, date and time
- Messages you received directly
- List of witnesses who saw it themselves
- Evidence log in the form in Section 5
Time and status
- Date you first learned, and count one year
- What happened after you found out, for example whether you stayed together
- A first decision: divorce, or a claim against the third party only
What not to do
- Do not log in to your spouse's accounts or devices, install tracking apps or post accusations
Further reading
- Divorce, marital property and child custody — overview of a divorce case
- Suspect your spouse is moving money or transferring assets before divorce — finding asset information lawfully
- Online defamation in Thailand — where the victim becomes the accused
- Maintenance and custody — the children's side
- What to agree in a prenuptial agreement — terms that change how property is divided
- Spousal alimony after divorce versus child support — support after divorce
Start with the goal, keep only evidence obtained lawfully, write down the date you learned, and do not let more than a year pass.
We help set the goals of the case, review the evidence you have and bring the divorce or compensation claim. For an initial consultation, call +66 92 254 2045 or send us the details. See also our family law practice.
This guide is published by Suwanvara Law Firm — a Khon Kaen law firm established in 1986. General information only, not legal advice on a specific matter.
