Which king had Daniel thrown into the lions' den?

Answer: Darius

King Darius signed the decree and, though distressed, gave the order to throw Daniel into the den of lions.

“Therefore King Darius signed the written decree. So the king gave the order, and they brought Daniel and threw him into the den of lions. The king said to Daniel, “May your God, whom you serve continually, deliver you!””— Daniel 6:9, 6:16 (BSB)