  For over a century, the Hall effect, a transverse effect under out-of-plane
magnetic field or magnetization, has been a cornerstone for magnetotransport
studies and applications. Modern theoretical formulation based on the Berry
curvature has revealed the potential that even in-plane magnetic field can
induce anomalous Hall effect, but its experimental demonstration has remained
difficult due to its potentially small magnitude and strict symmetry
requirements. Here we report observation of the in-plane anomalous Hall effect
by measuring low-carrier density films of magnetic Weyl semimetal
EuCd$_2$Sb$_2$. Anomalous Hall resistance exhibits distinct three-fold
rotational symmetry for changes in the in-plane field component, and this can
be understood in terms of out-of-plane Weyl points splitting or orbital
magnetization induced by in-plane field, as also confirmed by model
calculation. Our findings demonstrate the importance of in-plane field to
control the Hall effect, accelerating materials development and further
exploration of various in-plane field induced phenomena.
