Short answer. Yes. The Revised Penal Code punishes a private individual who seizes another's papers or letters in order to discover his secrets. Revealing what was found carries the heavier penalty; keeping quiet about it is still an offence. Spouses and parents of minors are expressly excepted.
What the law says
shall be imposed upon any private individual who in order to discover the secrets of another, shall seize his papers or letters and reveal the contents thereof
Revised Penal Code, Article 290 — Seizing Correspondence To Discover Secrets. Read the full provision →
Purpose is the heart of the offence
The words in order to discover the secrets of another do the real work here. This is not a rule against handling someone else's mail; it is a rule against prying. Picking up a letter delivered to the wrong address, or opening an envelope in the honest belief that it was yours, does not fit the article because the purpose is missing. What is punished is the deliberate seizure of another person's papers or letters in order to learn what he wanted kept private. The papers need not contain anything scandalous — the protected interest is the privacy of the correspondence, not the quality of the secret.
Two penalties, depending on whether you talked
The article grades the offence by what the intruder did next. Where he seized the papers and revealed the contents, the penalty is prisión correccional in its minimum and medium periods together with a fine not exceeding ₱100,000. Where he did not reveal the secrets, the penalty drops to arresto mayor with the same fine ceiling. The lower bracket is easy to overlook and worth stating plainly: a person who read the letters and told nobody has still committed a crime. Both peso figures are those set by Republic Act No. 10951 (2017), which revised the fines throughout the Revised Penal Code; older reproductions print the 1930 amounts of ₱500 and are out of date.
Who is outside the article
Two exceptions are written into the text. It does not apply to parents, guardians or persons entrusted with the custody of minors with respect to the papers or letters of the children under their care, and it does not apply to spouses as regards each other's papers or letters. A husband reading his wife's letters is not committing this offence. Note also the opening words: the article speaks of a private individual. A public officer who does the same thing is dealt with under separate provisions on officials who open documents or interfere with correspondence, which carry disqualification from office as well.
Email, phones and the modern equivalents
This provision speaks of papers and letters, and it is not the law most people actually need today. Intercepting private communications is separately governed by the Anti-Wiretapping Act (RA 4200); accessing someone's account, device or messages without authority falls under the Cybercrime Prevention Act of 2012 (RA 10175); and misuse of personal information can engage the Data Privacy Act of 2012 (RA 10173). Those laws carry heavier consequences than this article and reach conduct it never contemplated. The constitutional guarantee of privacy of communication and correspondence sits behind all of them. If you are on either side of such a dispute, preserve the devices and records untouched and take counsel before acting.