Ultrahigh performance concrete (UHPC) is an advanced cementitious material which has excellent mechanical and durability properties, making it appropriate for the rehabilitation of concrete structures. This paper investigates the usage of a thin layer of UHPC overlaying a normal strength concrete (NC) deck. The behavior of the interface connection will have a significant impact on the overall structural and durability performance of the UHPC-NC composite deck system. An integrated experimental and analytical study was conducted to understand the influence of several variables, such as normal concrete strength, interface roughness, and curing condition on the shear transfer behavior across the interface between UHPC and NC. The laboratory testing was performed in two phases, including slant shear testing in Phase-I and flexural testing of composite deck specimens in Phase-II. A total of sixty test units with five different surface textures and three different concrete strengths were loaded to failure in Phase-I. Four 197 mm (7.75 in.) thick, 0.61 m (2 ft) wide, and 2.74 m (9 ft) long deck specimens with 38 mm (1.5 in.) thick UHPC overlay and different interface texture were tested to failure in Phase-II. The slant shear test results demonstrated that the shear transfer across the interface is adequate for overlay applications for all textures with roughness equal or more than 3 mm (0.12 in.) regardless of concrete strength, which was later confirmed by the composite deck specimen tests. The interface bond strength was not affected by curing conditions (heat treatment or ambient) used for UHPC. Current bridge design guidelines provide a conservative estimate for the UHPC-NC interface shear strength.