  We present a search for new members of the 300 km s$^{-1}$ stream (300S) near
the dwarf galaxy Segue 1 using wide-field survey data. We identify 11
previously unknown bright stream members in the APOGEE-2 and SEGUE-1 and 2
spectroscopic surveys. Based on the spatial distribution of the high-velocity
stars, we confirm for the first time that this kinematic structure is
associated with a 24$^{\circ}$-long stream seen in SDSS and Pan-STARRS imaging
data. The 300S stars display a metallicity range of $-2.17 < {\rm [Fe/H]} <
-1.24$, with an intrinsic dispersion of 0.21$_{-0.09}^{+0.12}$ dex. They also
have chemical abundance patterns similar to those of Local Group dwarf
galaxies, as well as that of the Milky Way halo. Using the open-source code
galpy to model the orbit of the stream, we find that the progenitor of the
stream passed perigalacticon about 70 Myr ago, with a closest approach to the
Galactic Center of about 4.1 kpc. Using Pan-STARRS DR1 data, we obtain an
integrated stream luminosity of $4 \times 10^3$ L$_{\odot}$. We conclude that
the progenitor of the stream was a dwarf galaxy that is probably similar to the
satellites that were accreted to build the present-day Milky Way halo.

Astrophysics of Galaxies