We report the discovery of thermal X-ray emission from the youngest Galactic supernova remnant G1.9+0.3, from a 237 ks Chandra observation. We detect strong K Delta *a lines of Si, S, Ar, Ca, and Fe. In addition, we detect a 4.1 keV line with 99.971% confidence which we attribute to 44Sc, produced by electron capture from 44Ti. Combining the data with our earlier Chandra observation allows us to detect the line in two regions independently. For a remnant age of 100 yr, our measured total line strength indicates synthesis of (1-7) X 10--5 M of 44Ti, in the range predicted for both Type Ia and core-collapse supernovae (SNe), but somewhat smaller than the 2 X 10--4 M reported for Cas A. The line spectrum indicates supersolar abundances. The Fe emission has a width of about 28,000 km s--1, consistent with an age of ~100 yr and with the inferred mean shock velocity of 14,000 km s--1 deduced assuming a distance of 8.5 kpc. Most thermal emission comes from regions of lower X-ray but higher radio surface brightness. Deeper observations should allow more detailed spatial mapping of 44Sc, with significant implications for models of nucleosynthesis in Type Ia SNe.