The longest dinosaur tooth on record belongs to the theropod dinosaur called Spinosaurus.

Spinosaurus is known for its massive sail-like structure on its back and its semi-aquatic adaptations. The teeth of Spinosaurus are conical and elongated, resembling those of crocodiles.

One exceptionally long tooth belonging to Spinosaurus was discovered during excavations in Morocco and measured approximately 29 centimeters (11.4 inches) in length. This tooth is believed to have come from the upper jaw of the dinosaur and represents an impressive adaptation for capturing and gripping prey.

The elongated teeth of Spinosaurus are indicative of its piscivorous (fish-eating) diet. This suggests that Spinosaurus likely hunted and fed primarily on aquatic animals, utilizing its long teeth to grasp and hold slippery prey such as fish or other aquatic creatures.

The discovery of such long teeth in Spinosaurus provides valuable insights into its feeding behavior and adaptations for an aquatic lifestyle. It also contributes to our understanding of the ecological diversity and evolutionary adaptations of dinosaurs.

The remains of spinosaurus can be found in the Field Museum. More info from the link:

https://www.fieldmuseum.org/exhibitions/spinosaurus

It is important to note that tooth size and shape can vary among individuals and even within species. Some other dinosaurs, such as Tyrannosaurus rex, also had large and robust teeth, albeit not as long as those of Spinosaurus.

There is ongoing debate about the largest dinosaur tooth, as there are reports of Tyrannosaurus having 12 inch teeth. More info from the link:

https://discoverrings.co.uk/how-many-teeth-did-a-t-rex-have/