The earth is round first discovered by

It isn't exactly known who discovered it first, but the earliest claim of earth being round rather than flat was posed by Pythagoras sometime around 500 B. C. He argued that if the moon was round the Earth should be as well.

After that, his claim was proven true by a man named Anaxagoras at around 500-430 B. C when he brought up solar and lunar eclipses. The shape of Earth's shadow on the moon was also used as evidence that the Earth was round.

And finally, in 350 B. C the great Aristotle declared that the Earth was a sphere based on constellations you can see if you travel farther from the equator. Over the next hundred or so years, Aristarchus and Eratosthenes actually measured the size of the earth.

Short answer: The Greeks