In this paper, we introduce an interior penalty tensor-product preconditioner for the implicit time integration of discontinuous Galerkin discretizations of partial differential equations with second-order spatial derivatives. This preconditioner can be efficiently formed using a sum-factorized Lanczos algorithm for computing the Kronecker-product singular value decomposition of the diagonal blocks of the Jacobian matrix, and can be applied efficiently using a simultaneous triangularization procedure. In two spatial dimensions, the computational complexity for the overall method is linear per degree of freedom, which is the same as that of a sum-factorized explicit method. This preconditioner exactly reproduces the block Jacobi preconditioner for certain special cases, and compares favorably with the block Jacobi preconditioner for a range of test problems, including viscous compressible flow over a circular cylinder. This preconditioner shows greatly improved performance when compared with a Kronecker-product preconditioner that only incorporates first-order terms.