A Madrid man is missing in London after he used his skateboard against one of the jihadists, who was stabbing a woman, Spanish media report.
Ignacio Echeverría, 39, saw the attack unfolding at London Bridge on Saturday night, and rushed to help the woman.
Friends with him at the time told his family what had happened.
Reports say Mr Echeverría is not listed among the dead. His sister has gone to London hospitals, but there is no news of him being among the injured.
In a message on Facebook his father Joaquín Echeverría urged members of the public to help find his son, posting details of Ignacio.
At least one other Spaniard is in a London hospital, lightly injured after the terror attack at London Bridge.
Reports say Ignacio Echeverría came upon the scene at London Bridge while cycling back from a park with two friends, with whom he had been skateboarding.
In a Facebook post, his father said: "They saw him lying on the floor on the sidewalk after defending someone with his skateboard."
Apparently Ignacio did not have ID on him at the time. The Spanish embassy and HSBC Bank, his employer in London, are helping to search for him.
On Saturday night three men rammed their van into pedestrians on London Bridge, then went on a stabbing rampage at nearby bars and restaurants, before police shot them dead.
They killed seven and wounded dozens more, of whom 21 are now critically ill in hospital.
The attack has been claimed by so-called Islamic State. At least one of the killers had shouted "this is for Allah!" during the attack, witnesses said.