A modular staircase frame is lightweight, adjustable, and may be brought to a worksite in pieces to be assembled. The modular staircase frame includes two opposing sets of laterally spaced blades, which may be adjustably connected to form laterally spaced ascending frame members. The opposing sets of blades may include a first pair of blades and a final pair of blades to anchor the frame to a first and second surface, respectively. The blades have openings for adjustable attachment of adjacent blades. The frame may also include stiffeners for added structural support. The opposing sets of laterally spaced blades may further include one or more corner blades for changing the direction of the staircase. Once the frame is installed, treads (temporary or final) can be removably installed on the frame to create a staircase.