Dagger Island is a Caribbean island that the Cooper Gang traveled to during the Dead Men Tell No Tales episode in Sly 3: Honor Among Thieves. The island itself is separate from Blood Bath Bay, being "many leagues" from it. The island is indigenous to purple near-sighted lizards.
At some point during their prime, two pirates named Black Spot Pete and Cantankerous Tim robbed a treasure hunter named Reme Lousteau of his loot and diving gear. They subsequently buried the stolen items in Dagger Island and created a treasure map for them.
Decades later, a young captain named LeFwee stole the map from a now elderly Pete, placing it in Skull Keep. Meanwhile, Reme's grandson, Dimitri Lousteau, tasked the Cooper Gang with recovering the diving gear. After Sly Cooper used a pirate disguise to pose as Cantankerous Tim to get information on the diving gear from Pete, the gang realized that they would need to take the map from Skull Keep and steal a ship to travel to Dagger Island. After successfully doing so, the gang found the loot and diving gear on the island, with Dimitri keeping the gear. However, they were then greeted by Captain LeFwee and two of his guards, who took Penelope hostage and forced the gang to leave the rest of the treasure and sail away from the island. Meanwhile, LeFwee took the treasure and returned to Blood Bath Bay with Penelope in tow. The Cooper Gang eventually came up with a plan to save Penelope from LeFwee, a plan that was impossible without the diving gear that was buried on Dagger Island.