In the last few years, the flat slab system has become widespread for constructing multi-story buildings in many countries due to its simplicity and fast construction. However, the demand for the strengthening of reinforced concrete is a critical element in existing structures due to design or construction errors, and changes in the building function become a challenging area to develop and extend the durability of structures. In the reinforced concrete structure, flat slabs are the most critical element that requires strengthening because of exhibition punching shear failure. There are many materials and techniques used in the strengthening field. Due to well-known superior mechanical properties in strength and durability, ultra-high-performance fiber-reinforced concrete becomes the early relevant substance in the strengthening field. This study highlights the behavior of ten small-scale specimens of reinforced concrete flat slabs with different concrete grades, strengthened against punching shear by ultra-high performance fiber reinforced concrete strips in different distribution patterns at punching shear critical sections. The strips are jointed to the tensile slab surface through adhesive epoxy material. The outcomes indicated that the ultra-high performance fiber reinforced concrete strengthening strips enhanced the punching shear resistance of the normal strength concrete slabs, up to 53.1%, 16.63%, and 16.5% for different concrete grades examined in the study for flat slabs, which are 20.8, 32.6, and 43.3 MPa, respectively. This improvement in punching shear resistance was obtained by enhancing slab thicknesses and widening the resisting area for punching shear at critical sections. In addition, the strengthening technique transforms the failure's mode of slabs from brittle to ductile.