WARNING: CONTAINS GRAPHIC AND UPSETTING CONTENT
The discovery was made along the coast of Almeria in Andalucia, with the Spanish Civil Guard saying it has launched an investigation and hopes arrests will be made.
Authorities believe the deep cuts must have been 'produced by man', as some of the markings spelled out words or initials.
Animal rights campaigners also say they believe the culprit may have experience in 'dismembering' dolphins, or other marine animals.
One of the dead dolphins had been found on the beach of Cala Príncipe in San José, Níjar.
Volunteers from animal welfare group Equinac tried to move the animal, but realised its head had been severed from its body.
The dolphin's head was found on the shoreline nearby.
Equinac confirmed the cuts had not been the result of an accident with a boat or rocks, issuing a stern warning on social media to whoever is responsible.
The post said: "This message goes to you, the person who is cutting dolphins and decapitating them.
"Bear in mind that the Civil Guard of Almeria is conducting an investigation and they are very good.
"Neither the Almería Civil Guard, nor Equinac, are ignorant. We know the cuts are being made by someone who has experience in dismembering these, or other, marine animals.
"But, in addition, we have in Almería an exceptional Civil Guard, both Seprona, GEAS, Maritime Service, OPC and the Chief Colonel himself who, beyond their functions as guarantors of the laws that protect fauna and the environment take it very, very seriously, and with great determination!"
The post continued to express the group's gratitude towards authorities for their ongoing collaboration.
It said: "At Equinac we are very grateful and very proud of the collaborative work we have been doing for years with the Civil Guard Command of Almeria, and many cases have been taken to the same judgments thanks to them, because marine animals, like terrestrials, also have rights and laws that protect them."
Directly addressing the culprit once again, the message ended by saying: "If you go on, we will too, until we catch you."
Chosen for YouChosen for You
Most Read StoriesMost Read