Sinclair died last month, aged 73, at HMP Glenochil in Alloa, Clackmannanshire.
It is understood he had suffered from a series of strokes.
His body was cremated at Falkirk Crematorium last Wednesday morning before the facility opened, with no ceremony, flowers or music.
The ashes were returned to Clackmannanshire Council, which said they were “disposed of at sea”.
The information about Sinclair’s remains were published on the Clackmannanshire Council website, which also revealed the cremation cost of £1,150.
He was convicted of four killings, including the 1977 World’s End murders, but was suspected of killing four more women in Glasgow the same year.
He had been in prison since 1982 after being convicted of a series of rapes and indecent attacks on children.