The 35-year-old, who is from Villaescusa, was taking photos while walking along a popular hiking path between his hometown and Izara when he saw movement in the bushes, reported El Diario Montanes.
Thinking it must be some kind of animal, he prepared his camera to take a picture when he saw it was an adult bear coming after him.
The man tried to shield himself as the bear attacked him and tore up the man's arm with its claws. The bear then ran back into the bushes.
Another man from Villaescusa found the injured man trying to return home and helped him get to emergency services.
He was first taken to Hospital Tres Mares in Reinosa before being transferred to Valdecilla Hospital in Santander to undergo surgery by a trauma unit.
"All indications are that he experienced a rapid charge by an animal that felt threatened and was trying to get rid of the threat in some way. This is how bears behave," said Brown Bear Foundation president Guillermo Palomero to El Diario.
Experts said the area where the man was attacked was very popular among hikers because of its beauty, but it's also quite popular among bears. Palomero said that there are more and more bears going there and last year they found for the first time in the area a female bear with cubs.
"There are very few cases like this and, as in all cases, this happened because the bear felt threatened," Palomero said.
This article has been updated to reflect new information about the victim's age.