Short answer. Yes. The legitimate son or daughter of one spouse may be adopted by the other spouse, which is exactly the stepparent-adoption situation. But the process itself has changed: adoption is no longer filed as a court petition. Since 2022, domestic adoption runs through an administrative process at the National Authority for Child Care.
What the law says
The legitimate son/daughter of one spouse by the other spouse
RA 8552, Section 8 — Who May Be Adopted. Read the full provision →
Stepparent adoption is squarely covered
The law on who may be adopted lists several categories, and one of them is precisely your situation: the legitimate son or daughter of one spouse by the other spouse. Your stepchild being your spouse's legitimate child from a previous relationship, rather than a child of the two of you together, does not take the case outside this category. Stepparent adoption of a spouse's legitimate child is one of the recognized paths to adoption, alongside adopting a minor who has been declared available for adoption, an illegitimate child seeking legitimacy status, an adult who was raised as one's own since minority, a child whose earlier adoption was rescinded, and a child whose parents have died. The old statute also carved this situation out of its general rule that husband and wife must adopt jointly: a spouse adopting the legitimate child of the other could do so alone, rather than through a joint petition by both spouses.
The procedure has since moved out of the courts
The statute describing who may be adopted was originally enacted as Republic Act No. 8552, which set up domestic adoption as a court petition. That law has since been repealed in its entirety and replaced by Republic Act No. 11642, enacted in 2022. Under the newer law, domestic adoption is no longer initiated by filing a case in court. It is instead handled through an administrative process before the National Authority for Child Care. Anyone relying on the old court-petition procedure described in the original 1998 law is following a process that no longer applies.
What stays the same, and what to check next
The substantive rule on who qualifies for adoption, including the stepparent category, is what carried forward; it is the forum and the procedure that changed. Because the current law now runs the process administratively rather than judicially, the specific requirements, supporting documents, and steps involved are set by the newer statute and its implementing rules rather than by the 1998 law's original provisions. Anyone pursuing a stepparent adoption today should confirm the current requirements under Republic Act No. 11642 rather than relying on older descriptions of court-based adoption proceedings.