Dulce de membrillo and (Portuguese) mermelada are the same thing. In fact, in Gallego, dulce de membrillo is called marmelo, much closer to the portuguese word

Dulce de membrillo and (Portuguese) mermelada are the same thing. In fact, in Gallego, dulce de membrillo is called marmelo, much closer to the portuguese word for it. In Catalunya it's "codonyat" (the quince itself is called codony in catalan, and codoña in some other parts of Spain). I always thought that the English "marmalade" and the Spanish "mermelada" had a somewhat shared history, since in both cases, the portuguese meaning is lost and it picked up a similar meaning (jam type preserve). It's probably because the process of creating a jam is quite similar to that of creating a quince cheese, but with different fruits. Perhaps at some point, the word became a description of the technique, and not of the particular ingredients used.