We have developed a chlorine based dry etching process for nanopatterning the ferromagnetic oxide La0.7Sr0.3MnO3 (LSMO). Large arrays of millions of identical structures have been fabricated from thin LSMO films by electron-beam lithography and reactive ion etching. SQUID magnetometry demonstrates that patterned nanostructures with lateral dimensions down to 100 nm retain their full magnetization and the Curie temperature of the bulk layer. In addition, their shape anisotropy is sufficient to overcome the crystalline anisotropy of the bulk. High resolution scanning transmission electron microscopy shows that crystallinity is preserved even at the edges of the nanostructures.