USA move to Spain

I guess you have visited here :-

https://ec.europa.eu/home-affairs/what- … ication_en

That looks interesting. On first glance not sure it applies she has been living in USA married to me for about 10 years and of course I am just a US citizen.

If your wife becomes resident in spain then you can apply for family reunification

When we apply for family reunification in Spain we just need our USA marriage certificate? What agency in Spain do we go to to do it?

You will want to double check this but I believe all that you need* is to
1) Get your marriage certificate "apostilled".  I have obtained LOTS of apostilles for documents used in Europe through a service in the US; there are quite a few of them.  Not very expensive, I think it was $100/document.  This saves a lot of trouble
2) After that, you need to get it stamped by the Spanish consulate in the US

Sounds complicated, it isn't.  People do this all the time. It does take 2 to 3 weeks to complete however.

*"All you need"; that is with respect to using the marriage certificate.

There's more requirements for the family reunification process which JohnCar references, however.  I went through this for my adopted daughter, and while straightforward there's some requirements that may leave you scratching your head.  Nothing that is unsurmountable.

Good luck and regards

See link below. 
Note that there are quite a few ways that one can exercise a "family reunification" visa.  In our case, we are expats with residence permit in Spain.  This is a different (more demanding) case than yours.  Look further down in the link to the case of "Eu resident sponsoring someone" as that is closer to your case

https://www.expatica.com/es/moving/visa … on-422591/

"I have obtained LOTS of apostilles for documents used in Europe through a service in the US; there are quite a few of them.  Not very expensive, I think it was $100/document.  This saves a lot of trouble
2) After that, you need to get it stamped by the Spanish consulate in the US"

Those 'apostille' paperworks also applies to US/ European University degrees- Undergraduated degrees/certificates
in order to get it valid - equaly scholar valued- in Spain.

Nevertheless have to point out that there are international diplomas not-officially recognized by the 'Spanish Education Board' -

Those equivalences with Spanish degrees aren't reassured even those degrees/certificates were legally sowrned-translated into Spanish by an accreditated translator.