Crown Princess Letizia, 33, gave birth to Leonor, who weighed 7lb, early yesterday by Caesarean section in Madrid's Ruber International clinic.
When it won power last year, the Socialist government pledged to bring greater sexual equality. Reforming the constitution was a cornerstone of this policy.
Spain is one of the last European countries where males have priority over females in line to the throne. La Infanta Leonor is second in line to the throne after her father Crown Prince Felipe, but can only become queen if she has no brothers.
Denmark, the only other European country with similar rules of succession, is planning reform.
Crown Prince Felipe said the birth of his daughter was "significant from a political and constitutional point of view".
Spain's government promised yesterday to reform the constitution by 2008. Since winning power, the Prime Minister, Jose Luis Rodriguez Zapatero, has changed the law to crack down on domestic violence, with longer jail terms for offenders and better protection for women. His government also plans a "housework" law to force men to do more of the daily chores.
Spain's birth rate is one of the lowest in Europe because most women put off giving birth until they are in their thirties. Most have short-term, uncertain work contracts, which do not help budgeting for a family. Only one High Court judge is female.Reuse content