Resen was, according to Genesis 10:11, a city founded by Nimrod son of Cush.
Resen is stated, according to Genesis 10:12, to have been located between Nineveh and Calah and became a great city. Its exact location is today unclear. According to Genesis, it is within the vicinity of ancient Assyria, which should place it in Iraq, but the exact location is not known. Some have identified it with the ruins of Yassin tepe and others with Karamles.
From Genesis chapter 10:
8 Cush was the father of Nimrod, who grew to be a mighty warrior on the earth. 9 He was a mighty hunter before the LORD; that is why it is said, "Like Nimrod, a mighty hunter before the LORD." 10 The first centers of his kingdom were Babel, Erech, Akkad and Calneh, in Shinar. 11 From that land he went forth, where he built Nineveh, Rehoboth-Ir, Calah 12 and Resen, which is between Nineveh and Calah; that is the great city.