A complete birational classification of algebraic tori with a biquadratic splitting field is obtained in this paper. It is shown that any torus of the indicated type is birationally equivalent to a direct product of $n$ copies $(n\geqslant0)$ of a special three-dimensional torus $\mathscr I$ and an affine space $\mathbf A^m$ . An affirmative answer to one of Zarisskii's conjectures is also obtained for tori of this type. Up till now a birational classification of tori has been known only in the case of a metacyclic splitting field (i.e. in the case where all Sylow subgroups of the Galois group of the splitting field are cyclic). Bibliography: 12 titles.