The tropicalization of a linear space over a non-archimedean field is a tropical linear space. In this paper, we present a method for computing the tropicalization of any lattice over a valuation ring. The resulting tropical semimodule is the support of a polyhedral complex constructed from a certain multilinear polynomial we call the entropy polynomial. The key idea in our argument is the tropicalization of Haar measures on lattices over local fields.