We report on the filament of ionized gas found at the NE periphery of M87, the second brightest elliptical galaxy in the Virgo cluster. The object lies at 3.16 arcmin (15.7 kpc) projected distance of the nucleus of M87, and it coincides in position with the Eastern radio lobe of M87. Long slit spectroscopy confirmed the gaseous nature of this object, showing Hbeta, [OIII], [OI], [NII], Halpha and [SII] lines, without any underlying continuum. The redshift of the object, V = 1170 +/- 25 km s-1, agrees with the velocity of M87 itself (V = 1280 km s-1), and with the bulk of the ionized material found near the center of M87. The Halpha flux of this filament corresponds to 1% of the ionized material in M87. The radio-optical coincidence together with the spectral characteristics suggest that the filament is ionized by a shock. Based on observations taken at INT and OHP. The INT is operated by the I.N.G. at the Spanish Observatorio del Roque de Los Muchachos of the I.A.C.. The OHP is operated by the French CNRS.