3D integrated circuit packages with window interposers and methods to form such semiconductor packages are described. For example, a semiconductor package includes a substrate. A top semiconductor die is disposed above the substrate. An interposer having a window is disposed between and interconnected to the substrate and the top semiconductor die. A bottom semiconductor die is disposed in the window of the interposer and interconnected to the top semiconductor die. In another example, a semiconductor package includes a substrate. A top semiconductor die is disposed above the substrate. An interposer is disposed between and interconnected to the substrate and the top semiconductor die. A bottom semiconductor die is disposed in a same plane as the interposer and interconnected to the top semiconductor die.