Composite sandwich panels with reinforcement ribs or webs are being increasingly considered for structural applications in civil engineering. Such panels are prone to creep when subjected to significant permanent loads and therefore the effects of this phenomenon must be duly accounted for in their design. Data regarding the viscoelasticity of sandwich panels and their constituent materials is still scarce and often cannot be directly used for creep predictions in design practice. To address this issue, this paper presents the experimental assessment and the analytical modeling of the viscoelastic response of two types of sandwich panels, with and without reinforcement ribs. Panels comprising glass fiber-reinforced polymer (GFRP) faces, cores of polyurethane (PU) foam, and longitudinal GFRP ribs are considered. The ribs increased the flexural strength and stiffness of the panels by a factor of two, while providing a threefold reduction in their creep compliance. A composite creep modeling (CCM) approach is assessed, inputting independently determined viscoelastic properties of the individual materials into Timoshenko beam theory to estimate the creep response of the full-scale panels. The predicted and experimental creep curves presented good agreement for different load levels, attesting the CCM's adequacy for the analysis of creep in sandwich panel design. (C) 2016 American Society of Civil Engineers.