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. The plurality of corner support columns have a second variable size depending on the number of the plurality of modular stair units to be stacked in the stairwell. A first stair runner removably connects to the mid-level landing and connects a bottom level to the mid-level landing. A second stair runner removably connects to the mid-level, rectangular landing and the upper, rectangular landing to connect the mid-level, rectangular landing to the upper, rectangular landing. Each of the plurality of modular stair units are organized such that a bottom modular stair unit is placed on a foundation and each successive modular stair unit of the plurality of modular stair units are stacked on top of and connected to a lower modular stair unit.