Do Spanish tax residents need to declare UK income?

Often, yes. Spanish tax residents are generally assessed on worldwide income, although the treaty and foreign-tax-credit rules affect which country taxes an item and how double taxation is relieved. UK tax already deducted does not by itself remove a Spanish reporting obligation.

This guide provides general information for planning purposes. It does not constitute legal, tax, financial, immigration or medical advice. Always confirm decisions with a qualified specialist authorised to advise for your circumstances.