Virgin Holidays have stopped selling tickets to shows where marine animals are kept captive.
The firm announced that its ended partnerships with five such venues including SeaWorld and Discovery Cove.
Virgin founder Richard Branson said they would now only "work with suppliers that don't take sea cetaceans from the wild".
Amusement parks like SeaWorld have been criticised for their treatment of dolphins and killer whales.
SeaWorld have responded to the decision by saying that activists "mislead and manipulate marine mammal science to advance their agendas".
In a statement, they said "no company does more to protect marine mammals and advance cetacean research, rescue and conservation."