► We combine IR (VIRTIS) and smm/mm (MIRO) data to constrain the thermal properties of Asteroid Lutetia. ► VIRTIS dayside temperature measurements in excess of 240K indicated thermal inertias less than 20. ► MIRO 2-channel polar night measurements indicated a Lutetia thermal inertia increasing with depth. ► Comparisons of model predictions with VIRTIS dayside temperatures indicated roughness effects. ► Addition of a model of 50% coverage of mini-craters resolves the model vs. measurement offsets. The European Space Agency’s Rosetta spacecraft is the first Solar System mission to include instrumentation capable of measuring planetary thermal fluxes at both near-IR (VIRTIS) and submillimeter–millimeter (smm–mm, MIRO) wavelengths. Its primary mission is a 1year reconnaissance of Comet 67P/Churyumov–Gerasimenko beginning in 2014. During a 2010 close fly-by of Asteroid 21 Lutetia, the VIRTIS and MIRO instruments provided complementary data that have been analyzed to produce a consistent model of Lutetia’s surface layer thermal and electrical properties, including a physical model of self-heating. VIRTIS dayside measurements provided highly resolved 1K accuracy surface temperatures that required a low thermal inertia, I<30J/(Km2s0.5). MIRO smm and mm measurements of polar night thermal fluxes produced constraints on Lutetia’s subsurface thermal properties to depths comparable to the seasonal thermal wave, yielding a model of I<20J/(Km2s0.5) in the upper few centimeters, increasing with depth in a manner very similar to that of Earth’s Moon. Subsequent MIRO-based model predictions of the dayside surface temperatures reveal negative offsets of ∼5–30K from the higher VIRTIS-measurements. By adding surface roughness in the form of 50% fractional coverage of hemispherical mini-craters to the MIRO-based thermal model, sufficient self-heating is produced to largely remove the offsets relative to the VIRTIS measurements and also reproduce the thermal limb brightening features (relative to a smooth surface model) seen by VIRTIS. The Lutetia physical property constraints provided by the VIRTIS and MIRO data sets demonstrate the unique diagnostic capabilities of combined infrared and submillimeter/millimeter thermal flux measurements.