The important document that King John signed to limit royal power in England was the Magna Carta, signed in 1215. This charter was a response to the king's oppressive rule and his heavy taxation, which angered nobles and barons. The Magna Carta established crucial principles such as the rule of law and the right to a fair trial, asserting that the monarchy was not absolute and that subjects had certain rights. This document laid the groundwork for constitutional governance in England, influencing later democratic systems and legal thought, and is often heralded as a symbol of liberty and justice.