Angela Millington, 33, from Southend, Essex, had not been seen since February 2014. Her remains were found on Foulness Island four months later.
Two men, one aged 67 and the other 51, were arrested and bailed on suspicion of her murder.
Crimestoppers is offering the reward for information to find her killer.
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"This case is a bit of a mystery and it is unknown what ordeal Angela had gone through that resulted in her death," the charity's Antonia Litten said.
"I would ask anyone who might know who was responsible to contact Crimestoppers anonymously."
A £10,000 reward is being offered for information about the suspected murder of a homeless woman whose bones washed up on an island.