Sayhad or Ramlat as Sab'atayn (arabic رملة السبعتين) is a desert region that corresponds with Northern Deserts of Modern Yemen (Al-Jawf, Marib, Shabwah governorates) and Southwestern Saudi Arabia (Najran province). Located at 15°42′N 46°20′E.
Ramlat as Sab'atayn occupies an area of about 26,000 square kilometers being roughly 100 × 240 km occupying relatively flat alternating dunes. The Ramlat as Sab'atayn includes part of what was known to medieval Arab geographers as Ṣayhad and it extends from al-Khawr to the edge of the Rub al-Khali desert.
The desert is located in the governorates of Yemen El-Jauf, Marib and Shabwa