Your result
Spain — Citizenship by Descent
Possible
We can place you as soon as two facts are settled, and both are usually findable in an afternoon.
Spanish descent turns on a narrow question: which is the nearest person in your direct line who held Spanish nationality, and where were they born. A parent who was Spanish when you were born means you are already Spanish and simply need to register. A parent who was Spanish from birth and born in Spain gives you a right of option with no age limit and no need to live in Spain. A grandparent gives no consular claim, but does cut naturalisation to one year of residence, and there is a job-search visa reserved for grandchildren to get you there. Further back than a grandparent, and descent runs out.
So the things to find are: that person's birth certificate, showing where they were born; and any evidence of the nationality they actually held — a Spanish passport, a consular registration, a naturalisation record in another country and its date. That last date matters more than people expect, because Spanish nationality could be lost by acquiring another one, and whether that happened before or after your birth changes the answer completely.
Send us what you have, including the parts you are unsure about. Half-complete family records are normal, and the gaps are often filled from Spanish registry records rather than from anything you hold.