The illiterate woman was detained by religious police in 2005 and allegedly beaten and forced to fingerprint a confession that she could not read.
Among her accusers was a man who alleged she made him impotent.
Human Rights Watch said that Ms Falih had exhausted all her chances of appealing against her death sentence and she could only now be saved if King Abdullah intervened.
The US-based group is asking the Saudi ruler to void Ms Falih's conviction and to bring charges against the religious police who detained her and are alleged to have mistreated her.
Its letter to King Abdullah says the woman was tried for the undefined crime of witchcraft and that her conviction was on the basis of the written statements of witnesses who said that she had bewitched them.
Human Rights Watch says the trial failed to meet the safeguards in the Saudi justice system.
The confession which the defendant was forced to fingerprint was not even read out to her, the group says.
Also Ms Falih and her representatives were not allowed to attend most of the hearings.
When an appeal court decided she should not be executed, the law courts imposed the death sentence again, arguing that it would be in the public interest.
Contact information for the Saudi Embassy:
Royal Embassy of Saudi Arabia 601 New Hampshire Avenue, NW Washington, DC 20037
Main Number: (202) 342-3800 Consulate/Visa Section: (202) 944-3126 Medical Office: (202) 342-7393 Commercial Office: (202) 337-4088