The ancient Greek philosopher who was sentenced to death for his ideas and teachings was Socrates. He was charged with corrupting the youth of Athens and impiety, primarily due to his method of questioning and dialogue that challenged traditional beliefs and norms. Socrates famously chose to accept his death sentence rather than compromise his principles or escape. His trial and execution in 399 BC had a profound impact on philosophy and ethics, prompting discussions about morality, justice, and the role of the individual in society. His teachings were later preserved by his student Plato.