Who was the first person to declare that the Earth was round?

Democritus in Greece and the Yellow Emperor of China...of course each country there was a probably first- so in theory such a person is debatable at best.

Eratosthenes, 200 b.c., within 5%.

Addendum: Eratosthenes was the first to calculate the circumference of the Earth. The idea that it is round is much older.

Pythagoras is possibly the earliest recorded mention of the idea, in the sixth century B.C. but it apparent from context that the idea is still older.

Eratosthenes famously calculated the Earth's circumference by showing that when the Sun shone directly down the wells at noon in Alexandria on a certain day, it was shining at an angle of 7 degrees to the vertical 50 miles away. So the circumference of the Earth was 50 X 360/7 miles. He is also now famous for sieving out prime numbers. In his day he was renowned for being second best at everything and got the nickname Beta. In the end, legend says he became depressed and starved himself to death.

But I digress. Another early Greek who clearly thought the Earth was round was Aristarchus of Samos (3rd C BC) who used the simple geometry of similar triangles to show that the Sun was much bigger than the Earth and Moon and concluded that the Earth went round the Sun. He also showed the stars were very very distant. Sadly his sensible system was replaced by the clumsy one of Ptolemy which was accepted into the Middle Ages. However, even this system had the Earth as round and is a myth that people of the Middle Ages generally thought that it asn't.
It is unknown who first declared that the Earth was round as it seemed to be apparent to ancient mariners who saw the changing horizon and to anyone who saw from the curved terminator of the Moon that celestial objects tended to be spherical.

Aristarchus of Samos not only declared the Earth round, but also suggested it orbited the sun. In order to make these hypotheses, Aristarchus measured the angles between the sun and the surface of the Earth in two different cities and found that the angles differed dramatically. From this measurement and other calculations, Aristarchus became the first person to prove that the Earth was round.

My information about Aristarchus comes from the book "The Handy Physics Answer Book" by P. Erik Gunderson (p. 10).

The Yellow Emperor knew this in 1800 BC- I think he wins!

First, Hamlet, you did not ask who was the first to prove, but the first to "declare" it was so.

It was already an ancient idea by the time of Aristarcus, and he proceeded on that assumption; therefore it had long since been declared. Also Aristarcus did not prove anything new; he never offered evidence that the earth was round as that was already accepted knowledge. Aristarchus never gave a size for Earth, but only the relative size of the Earth, Sun and Moon, and there he was quite wrong in his conclusions. His geometry was right, but his observations flawed. Aristarchus' real groundbreaking observation was that the Earth orbited the Sun.

Leaving aside the phrasing of the question, can you give a reference to the effect that Aristarchus proved the Earth was round? In the meantime, here are some to point out my contention:

http://www.varchive.org/ce/orbit/arisam.htm
www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Aristarchus.html no longer exists

In neither of these do I find any mention that Aristarchus was even interested in proving the Earth round. Neither, for that matter was Eratosthenes, he was trying to give an exact size for the Earth.

The Bible tells us in Isaiah 40:22 "It is He that sitteth above the circle of the earth, and the inhabitants thereof are as grasshoppers; that stretcheth out the heavens as a curtain, and spreadeth them out as a tent to dwell in;" This was written around 740 BC and is the oldest written proof.

A circle is not a sphere. It sounds to me as if Isaiah was a flat-earther, which is quite likely for his land and date.

https://en.wikipedia.org/wiki/Discworld