4587 Rees, provisional designation 3239 T-2, is a Mars-crosser and former near-Earth object on an eccentric orbit from the asteroid belt, approximately 2 kilometers (1.2 miles) in diameter. It was discovered during the second Palomar–Leiden Trojan survey on 30 September 1973, by Dutch astronomer couple Ingrid and Cornelis van Houten at Leiden, and Tom Gehrels at the Palomar Observatory in California. The assumed S-type asteroid has a rotation period of 7.9 hours and is likely elongated in shape. It was named after British astrophysicist Martin Rees.