Short answer. If the biological parents are not known, consent instead comes from the legal guardian or the government instrumentality with legal custody of the child, along with the other required consents such as the adoptee's own if age ten or older. Note: Republic Act No. 11642 (2022) now governs adoption.
What the law says
The biological parent(s) of the child, if known, or the legal guardian, or the proper government instrumentality which has legal custody of the child;
RA 8552, Section 9 — Whose Consent Is Necessary. Read the full provision →
The rule adjusts for exactly this situation
Section 9 does not require the impossible. Its clause on parental consent is written with a built-in condition: consent comes from the biological parent(s) of the child, if known. Where the parents are not known, the requirement shifts automatically to an alternative — either the legal guardian, or the proper government instrumentality that has legal custody of the child. The provision anticipates that biological parents will not always be identifiable and supplies a substitute rather than leaving the adoption unable to proceed.
Consent from the child's biological parents is only one requirement
Even where the parents are unknown and this substitute applies, Section 9 still requires consent from several other people, where applicable: the adoptee personally, if ten years of age or over; any legitimate or adopted children of the adopter and adoptee who are ten or older; any illegitimate children of the adopter living with the adopter and the adopter's spouse, if ten or older; and the spouse, if any, of the person adopting or being adopted. The parents' unknown status affects only the parental-consent clause, not these other requirements.
Why consent is required at all before this
Section 9 frames every one of these consents as coming only after being properly counseled and informed of the right to give or withhold approval. This counseling requirement applies across the board — it is not limited to the biological parents' consent, but frames how each required party, including a guardian or government instrumentality standing in for unknown parents, is expected to arrive at their decision.
This definition is not the current adoption procedure
This provision comes from a 1998 law that has since been overtaken. Republic Act No. 11642 (2022) repealed that law in its entirety and moved domestic adoption into an administrative process run by a national child-welfare authority, rather than the court-petition process this older law described. The underlying idea — that someone must stand in for unknown biological parents when consent is required — still reflects the kind of safeguard adoption law aims for, but the actual current procedure runs through Republic Act No. 11642, not the process described here.