Paul says that Adam was the first sinner.
Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned: ... Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come. Romans 5:12-14
The apocryphal / Deuterocanonical book of Ecclesisaticus / Sirach says it was Eve.
From the woman came the beginning of sin, and by her we all die. Ecclesiasticus 25:33
