Abstract: A modular stair system comprises a plurality of modular stair units. Each of the plurality of modular stair units are associated with a single story of a construction. An upper, rectangular landing comprises a pair of long members connected at each end to a pair of short members. A mid-level, rectangular landing comprises a first pair of members and a second pair of members interconnected at their ends. A first and second plurality of corner brackets interconnects an end of the members with an end of the members of the upper and mid-level landings. The first and second plurality of corner brackets have the variable size depending on the number of the plurality of modular stair units to be stacked in the stairwell. A plurality of corner support columns support the upper, rectangular landing at a top portion thereof and the mid-level, rectangular landing at a mid-portion thereof and removably connected to the upper, rectangular landing and the mid-level, rectangular landing.