If you´ve paid into the Spanish Social Security system, either as employee or as self employed Autonomo, in certain circumstances you can also be entitled to a complement, to bring the amount up to 100% of the amount for minimum pensions in Spain.
Depends on whether you have any other income (state pensions excluded) not exceeding a certain limit. For 2017 that´s 7.134 euros/year. The complement will be higher if you have a dependent spouse who is not in receipt of any state pension and your total income (pension excluded) doesn´t exceed the limit of 8.322 euros/year.
If you´ve become eligible to receive a state pension after 1.1.2013, you also need to be a resident in Spain to be able to receive the ´Complemento´and the extra payment will never exceed the absolute minimum for Spanish pensions – the non-contributive retirement or disability pension.
More info from the Seguridad Social website – in English
Sample of form to use to communicate extra income and residency requirement.
If you also have a state pension or pensions from other EU country(s), all those have to be added up, and the added sum will be taken into account when re-evaluating your pension to minimum standard.