The magnetic ordering of Pr in tetragonal PrBa2Cu2.7Zn0.3O7-y has been studied by neutron-diffraction measurements. In this Zn-doped system, where 15 at.% of the Cu atoms located in the CuO2-plane layers have been replaced by Zn atoms, the magnetic structure of Pr is different from that found in the undoped system. The basic magnetic structure of Pr in the present Zn-doped compound consists of a simple antiferromagnetic arrangement in the tetragonal ab plane with nearest-neighbor spins along the c axis aligned parallel (ferromagnetic). This contrasts with the structure found in PrBa2Cu3O7, where Pr spins are arranged antiferromagnetically along all three crystallographic directions. The ordering temperature is almost-equal-to 17 K, with a moment of 0.70+/-0.07mu(B) directed along the c axis. Although the doping of Zn changes the magnetic structure of Pr, its Neel temperature and moment remain unaltered. Our order-parameter measurements give a value of 0.49+/-0.09 for the critical exponent beta of the staggered magnetization, suggesting that the magnetic phase transition observed is three dimensional in nature.