We have experimentally studied a magnetopiezoelectric effect predicted recently for magnetic metals with low crystal symmetries. In EuMnBi2 with antiferromagnetic Mn moments at 77 K, dynamic displacements emerge along the a direction upon application of ac electric fields in the c direction and increase in proportion to the applied electric fields. Such displacements are not observed along the c direction of EuMnBi2 or EuZnBi2 with nonmagnetic Zn ions. As temperature increases from 77 K, the displacement signals decrease and disappear at about 200 K, above which electric conduction changes from coherent to incoherent. These results demonstrate the emergence of the magnetopiezoelectric effect in a magnetic metal lacking inversion and time-reversal symmetries.