Today, steel girder bridges with an overlaying concrete deck are typically designed and constructed as composite structures. This approach optimizes the utilization of materials and structural components. However, many existing steel–concrete bridges were originally designed without shear connectors at the steel–concrete interface. With the increasing demands of heavier traffic loads and a greater number of heavy load cycles, these bridges may sometimes require strengthening. One effective method to enhance their bending moment capacity is to develop composite action by installing shear connectors. One type of connector that is well-suited for postinstallation is the coiled spring pin. These coiled spring pins are inserted from beneath the bridge, passing through holes drilled in both the top flange of the steel structure and the concrete deck. This installation process can be executed seamlessly during ongoing traffic without causing traffic disruptions. This stands in contrast to postinstallation of welded-headed studs, where the pavement, water insulation, and concrete must be removed before the connectors are welded to the steel flange, followed by the application of new concrete and surface protection. This paper presents a monitoring project on a bridge strengthened with postinstalled shear connectors. The measurements were done both before and after the strengthening, which made it possible to evaluate the behavior of the nonstrengthened and strengthened structures. A verification with an finite-element model of the bridge with and without the shear connectors was made. The result from the measurements indicates that the steel girder and the concrete deck act as a composite section both before and after the strengthening at the tested load levels. The evaluation of the results also includes the vertical and longitudinal displacements between the concrete slab and the steel top flange, where the latter displacement is also denoted slip. The slip for the nonstrengthened bridge also indicates a full composite behavior. Due to the lack of shear connectors, other interlocking phenomena are most likely sufficient to achieve composite behavior at the tested load levels. Therefore, after the installation of the shear connectors, only a small reduction of the slip is noticed. Nevertheless, shear connectors significantly enhance the connection at the steel–concrete interface and are more reliable and robust, especially for heavier loads in the ultimate limit state.