Probably the most important Romanesque artwork in Spain, the Portal of Glory of Santiago de Compostela is essentially made in granite, the most abundant stone in the NW of Iberia, with the only exception of five marble pieces. Different hypothesis on the origin of these marbles have previously been proposed based on visual assessment or by directly assuming a local source. To shed light on their quarry provenance, a multi-analytical study was performed combining polarized-light optical microscopy, cathodoluminescence microscopy, XRPD, SEM-EDS and stable C and O isotopes. The comparison of the results with the available databases reveals the use of marbles from the Estremoz Anticline (Portugal) in the three exquisitely carved columns strategically placed in the central arcade, whereas the other two pieces are local marbles, illustrating a more complex consumption of this noble material than that initially expected.