Hi!
Great question. Generally, yes, retirement income is taxable in Spain if you are a tax resident of Spain. If you live in Spain for more than 183 days out of the year you are considered a tax resident. It's best to speak with Spanish tax expert to discuss your specific situation as they can beset inform you of what to expect.
ProSpain Consulting can answer questions about taxes: https://shehitrefresh.com/prospainconsulting
I hope this helps!
Best,
Cepee, IL Correspondent - Spain