Precast concrete floor systems represent a major component of the cost and weight of precast concrete buildings. Hollow-core (HC) planking is considered the most common precast concrete floor system for residential and commercial buildings because of their economy, light weight, structural capacity, and ease of production and erection. However, the high thermal conductivity of HC planks hinders their use in radiant heated floor and roof applications where thermal insulation is needed. This paper presents the development of a precast/prestressed concrete sandwich floor panel that consists of an internal wythe of insulation and two external wythes of concrete similar to those used in sandwich wall panels. The main difference between the sandwich floor and wall panels is the design of shear connectors between concrete wythes to achieve full composite action under ultimate loads while simultaneously having an adequate creep resistance under ustained loads and acceptable deflection under live loads. Truss-shaped glass fiber-reinforced polymer ties, known as NU ties, were used as shear connectors because of their structural and thermal efficiency. The proposed floor panels have comparable weight and capacity to HC planks while being more thermally efficient and can easily be fabricated in standard casting beds with typical equipment, thus eliminating the high initial investment required for HC production. This paper presents the design, detailing, production, and testing of three 7.9-m-long (26-ft-long), 1.22-m-wide (4-ft-wide), and 200-mm-thick (8-in.-thick) specimens with 50-mm-thick (2-in.-thick) composite topping. The three specimens represent two different panel designs. One specimen has solid concrete ends and was designed for a total superimposed service load of 9.6 kPa (200 psf), whereas two specimens are fully insulated and designed for a total superimposed service load of 4.8 kPa (100 psf). The performance of the three specimens under flexure and shear loadings indicated that the proposed sandwich floor panels can achieve full composite action and have satisfactory structural performance and acceptable deflection characteristics. DOI: 10.1061/(ASCE)SC.1943-5576.0000213. © 2014 American Society of Civil Engineers.