Falak-ol-Aflak (Persian: فلک الافلاک, lit. 'the sky of the skies') or Shapur Khast Castle (Persian: دژ شاپورخواست) is a castle situated on the top of a large hill with the same name within the city of Khorramabad, the regional capital of Lorestan province, Iran. This gigantic structure was built during the Sassanid era (224–651).

The Khorramabad River runs past the eastern and south-western side of the Falak-ol-Aflak hill providing the fortress some natural protection on those sides. Today, the western and northern sides of the hill are bordered by the residential districts of Khorramabad.