  We present the first realisation of a solitonic atom interferometer. A
Bose-Einstein condensate of $1\times10^4$ atoms of rubidium-85 is loaded into a
horizontal optical waveguide. Through the use of a Feshbach resonance, the
$s$-wave scattering length of the $^{85}$Rb atoms is tuned to a small negative
value. This attractive atomic interaction then balances the inherent
matter-wave dispersion, creating a bright solitonic matter wave. A Mach-Zehnder
interferometer is constructed by driving Bragg transitions with the use of an
optical lattice co-linear with the waveguide. Matter wave propagation and
interferometric fringe visibility are compared across a range of $s$-wave
scattering values including repulsive, attractive and non-interacting values.
The solitonic matter wave is found to significantly increase fringe visibility
even compared with a non-interacting cloud.
