[en] Optical and electronic properties of black phosphorus strongly depend on the number of layers and type of stacking. Using first-principles calculations within the framework of density functional theory, we investigate the electronic properties of bilayer black phosphorus with an interlayer twist angle of 90°. These calculations are complemented with a simple $\stackrel{\to <!--\; ???\; -->}{k}\cdot <!--\; ???\; -->\stackrel{\to <!--\; ???\; -->}{p}$ model which is able to capture most of the low energy features and is valid for arbitrary twist angles. The electronic spectrum of 90° twisted bilayer black phosphorus is found to be x-y isotropic in contrast to the monolayer. However x-y anisotropy, and a partial return to monolayer-like behavior, particularly in the valence band, can be induced by an external out-of-plane electric field. Moreover, the preferred hole effective mass can be rotated by 90° simply by changing the direction of the applied electric field. In particular, a $+$0.4 (−0.4) V ${\stackrel{˚<!--\; ??\; -->}{\mathrm{A}}}^{-<!--\; ???\; -->1}$ out-of-plane electric field results in a  ∼60% increase in the hole effective mass along the $\mathbf{y}$ ($\mathbf{x}$) axis and enhances the $m_{\mathbf{y}}^{\ast <!--\; ???\; -->}/m_{\mathbf{x}}^{\ast <!--\; ???\; -->}$ ($m_{\mathbf{x}}^{\ast <!--\; ???\; -->}/m_{\mathbf{y}}^{\ast <!--\; ???\; -->}$) ratio as much as by a factor of 40. Our DFT and $\stackrel{\to <!--\; ???\; -->}{k}\cdot <!--\; ???\; -->\stackrel{\to <!--\; ???\; -->}{p}$ simulations clearly indicate that the twist angle in combination with an appropriate gate voltage is a novel way to tune the electronic and optical properties of bilayer phosphorus and it gives us a new degree of freedom to engineer the properties of black phosphorus based devices. (paper)