  We compute the shear viscosity of two-flavor QCD plasma in an external
magnetic field in perturbative QCD at leading log order, assuming that the
magnetic field is weak or soft: $eB\sim g^4\log(1/g)T^2$. We show that the
shear viscosity takes a form $\eta=\bar\eta(\bar B)T^3/(g^4\log(1/g))$ with a
dimensionless function $\bar\eta(\bar B)$ in terms of a dimensionless variable
$\bar B=(eB)/(g^4\log(1/g)T^2)$. The variable $\bar B$ corresponds to the
relative strength of the effect of cyclotron motions compared to the QCD
collisions: $\bar B\sim l_{mfp}/l_{cyclo}$. We provide a full numerical result
for the scaled shear viscosity $\bar\eta(\bar B)$.
