This article explores the influence of construction joints on the stress–strain behavior of monolithic reinforced concrete floor slabs in frame buildings. A 200 mm thick slab without joints, placed within a 6 × 6 m bay frame structure, is used as the reference model. The study is based on eleven spatial models that differ in the location of the “cold” construction joint, while material properties remain constant: concrete of compressive strength class B35 and A500C reinforcement with a 12 mm diameter arranged in a 300 × 300 mm grid. The findings show that the presence of a construction joint can increase slab deflection by up to 1.26 times, depending on its position. The most effective joint location is in zones of minimum bending moments, as regulated by Russian construction codes, where its impact on the stress–strain state is minimal. The results can be applied in structural analysis and in refining regulatory approaches to the placement of construction joints in monolithic reinforced concrete floor slabs.