Scientists say their deaths are a mystery as it was not immediately clear what killed them.
Their carcasses were strewn along a 4-km (2.5-mile) stretch of Nungwi, said Narriman Jidawi, a marine biologist at the Institute of Marine Science in Zanzibar.
Narman Jidawi said it appeared the dolphins somehow became stranded when the tide went out Friday morning.
He said they did not starve to death and were not poisoned.
The bottleneck dolphins, which live in deep offshore waters, had empty stomachs.
This suggests they could have been disorientated and were swimming for some time to reorientate themselves.
The Indo-Pacific bottlenose, humpback and spinner porpoises, commonly known as dolphins, are the most common species in Zanzibar's coastal waters.
They are popular attractions for tourists visiting Zanzibar's famed spice islands where snorkelers can swim with the creatures as they frolic in the ocean.
The deaths are a blow to the tourism industry in Zanzibar, where thousands of visitors go to watch and swim with wild dolphins, said Abdulsamad Melhi, owner of Sunset Bungalows.
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Villagers, fishermen and hotel residents found the carcasses and alerted officials. Mussa Aboud Jumbe, Zanzibar's director of fisheries.
He went on state radio to warn the public against eating the dolphins' meat, saying the cause of death had not been determined.
But residents who did eat the dolphins' meat were all doing fine, Jidawi said.
In the US, experts were investigating the possibility that sonar from US submarines could have been responsible for a similar incident in Marathon, Florida.
In that incident in March 2005, 68 deep-water dolphins stranded themselves.
A US Navy task force patrols the East Africa coast as part of counterterrorism operations.
A Navy official was not immediately available for comment, but the service rarely comments on the location of submarines at sea.