Are you married in a foreign country and wish to register the marriage in Spain?
The main requirement to register your foreign marriage in the Spanish Civil Registry is that, at least, one of the spouses has the Spanish nationality.
Please note that if the marriage is not recognised in the country where it took place, it will never be recognised in Spain either.
Assuming that the marriage is valid in the country where it took place, it will have full effect.
It is not compulsory to do so, although it is highly recommended to register it in the Spanish Civil Registry.
Some consequences of NOT registering a foreign marriage in Spain
If you decide not to register your marriage in Spain, this means that there will be no record of the marriage in Spain.
Therefore, any spouse could marry again. This could involve serious problems with double bonds. It could even constitute a bigamy crime, categorised in Article 217 of the Spanish Criminal Code.
The fact that you cannot register your marriage in Spain (since neither of the spouses are Spanish) does not mean that you cannot exercise some rights, such as divorce or inherit from the other spouse (in case he/she passes away).
All of these matters are regulated by Law, specifically, the Spanish Civil Law, the European Regulation and the Hague Convention.
It is important to determine the international regulation regarding the applicable Law. This will depend on the nationality of the spouses.
Some benefits of registering a foreign marriage in Spain
Registering a marriage in Spain is important since, among other things, you can benefit from tax deductions in your annual income tax return [⧉ ES] (IRPF).
Likewise, by registering the marriage in Spain, the foreign spouse can apply for residency in Spain, and he/she may furthermore apply for Spanish nationality.