  Suppose that $\dot{G}$ is an unbalanced signed graph of order $n$ with
$e(\dot{G})$ edges. Let $\rho(\dot{G})$ be the spectral radius of $\dot{G}$,
and $\mathcal{K}_4^-$ be the set of the unbalanced $K_4$. In this paper, we
prove that if $\dot{G}$ is a $\mathcal{K}_4^-$-free unbalanced signed graph of
order $n$, then $e(\dot{G})\leqslant \frac{n(n-1)}{2}-(n-3)$ and
$\rho(\dot{G})\leqslant n-2$. Moreover, the extremal graphs are completely
characterized.
