Kylian Mbappe scored a very controversial penalty to give Real Madrid the lead a Santander Racing before Jude Bellingham and Yan Diomande effectively ended the game as a contest before the break.
Madrid traveled to El Sardinero needing a win to move back to within three points of La Liga leaders Barcelona, who they beat in the Clásico last week.
And they got off to a controversial start as they were awarded a penalty in the 11th minute when Mbappé fell in the box, appearing to be felled by Matteo Prati.
Replays suggested there had been no contact, but the decision was upheld by the VAR, allowing it Mbappe to break the deadlockthat is to say, he has now scored against all 24 teams he has faced in LaLiga.
Bellingham then made it 2-0, latching onto Arda Guler and rounding Julen Agirrezabala before slotting the ball home.
But it was Diomande who provided the pick of the group in the first half, the Ivorian unleashing an unstoppable volley into the top left corner from the edge of the box to make it three and all but ensure that Madrid will return to the capital with all three points.