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We present ~120,000 Spitzer/IRAC candidate young stellar objects (YSOs) basedon surveys of the Galactic midplane between l~255 deg and 110 deg, includingthe GLIMPSE I, II, and 3D, Vela-Carina, Cygnus X, and SMOG surveys (613 squaredegrees), augmented by near-infrared catalogs. We employed a classificationscheme that uses the flexibility of a tailored statistical learning method andcurated YSO datasets to take full advantage of IRAC's spatial resolution andsensitivity in the mid-infrared ~3-9 micron range. Multi-wavelengthcolor/magnitude distributions provide intuition about how the classifierseparates YSOs from other red IRAC sources and validate that the sample isconsistent with expectations for disk/envelope-bearing pre-main-sequence stars.We also identify areas of IRAC color space associated with objects with strongsilicate absorption or polycyclic aromatic hydrocarbon emission. Spatialdistributions and variability properties help corroborate the youthful natureof our sample. Most of the candidates are in regions with mid-IR nebulosity,associated with star-forming clouds, but others appear distributed in thefield. Using Gaia DR2 distance estimates, we find groups of YSO candidatesassociated with the Local Arm, the Sagittarius-Carina Arm, and theScutum-Centaurus Arm. Candidate YSOs visible to the Zwicky Transient Facilitytend to exhibit higher variability amplitudes than randomly selected fieldstars of the same magnitude, with many high-amplitude variables havinglight-curve morphologies characteristic of YSOs. Given that no current orplanned instruments will significantly exceed IRAC's spatial resolution whilepossessing its wide-area mapping capabilities, Spitzer-based catalogs such asours will remain the main resources for mid-infrared YSOs in the Galacticmidplane for the near future.

SPICY: The Spitzer/IRAC Candidate YSO Catalog for the Inner Galactic Midplane