An essential ingredient in modeling the response of lithium in an all-solid-state lithium battery is a theory for the large deformation elastic-viscoplastic response of lithium. In this paper we report on a such a theory. The material parameters for the theory have been calibrated using stress-strain data from direct tension tests on polycrystalline lithium specimens reported recently in the literature. The theory has been implemented as a user-material-subroutine in a finite element program. This simulation capability should be useful in the design and development of lithium metal solid-state-batteries. (C) 2019 The Electrochemical Society.