In a statement posted on Twitter she said: "With deep regret I have this morning written to the Prime Minister & tendered my resignation. I can no longer support Govt policy on Gaza."
Baroness Warsi has been vocal on Twitter about the conflict.
On July 24 she tweeted: "Can people stop trying to justify the killing of children. Whatever our politics there can never be justification, surely only regret #Gaza".
The 43-year-old worked for the Crown Prosecution Service and then set up her own legal practice before becoming involved in politics.