Soft X-ray emission from ~0.1 to 10 keV (~1-100 Å) provides unique diagnostics for high-temperature plasmas, but observations of the Sun in this energy range that are both spectrally and spatially resolved have been nearly non-existent. The Multi-Order X-ray Spectral Imager (MOXSI) is an instrument concept for a novel, slitless X-ray imaging spectrograph that will make crucial new measurements in this observationally-important wavelength range yet still fit within the limited resource constraints of a CubeSat. MOXSI utilizes a custom pinhole camera with a COTS, back-thinned CMOS sensor combined with a Chandra-heritage X-ray transmission diffraction grating to provide spatially-resolved, full-Sun imaging spectroscopy from ~1 to ~55 Å (~0.2-10 keV) with ~25 arcsec and ~0.25 Å FWHM spatial and spectral resolutions, respectively, and cadence of ~few tens of sec. MOXSI produces images akin to an “overlappograph,” with the 0th-order and dispersed images overlaid on the same detector; the dispersion direction is specifically oriented orthogonal to the latitudinal bands of solar activity to minimize source confusion. Additional pinhole apertures with custom entrance filters provide undispersed broadband filtergram images for additional source information. To mitigate motion-induced smearing, an on-board, real-time motion compensation system co-adds a series of frames for each integration period. MOXSI is one of the instruments of the proposed CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) mission concept, which will improve our physical understanding of thermal plasma processes and impulsive energy release in the solar corona, from quiet Sun to solar flares, and the impact of solar X-rays on Earth’s upper atmosphere.