<p>We study modular symmetries in non-supersymmetric heterotic string theories on toroidal backgrounds with Wilson line modulus, constructed by stringy Scherk-Schwartz compactification. In particular, we focus on a subgroup of the T-duality group 𝒪(<i>D</i> +16<i>, D,</i> ℤ) with <i>D</i> = 2 given by an outer automorphism of the Narain lattice, which can be mapped to the Siegel modular group Sp(4<i>,</i> ℤ). We classify the modular symmetries and a 𝒞<i>P</i>-like symmetry on <i>T</i>² and its orbifolds with symmetric and asymmetric orbifold twists. It turns out that the non-supersymmetric heterotic string theories only enjoy a part of modular symmetries in contrast to supersymmetric ones. Furthermore, the gauge symmetry is maximally enhanced at fixed points of modular symmetries on <i>T</i>² on which we analyze the vacuum structure of eight-dimensional tachyon-free vacua.</p>