We report the observation of gravitational waves from two compact binary coalescences in LIGO's and Virgo's third observing run with properties consistent with neutron star-black hole (NSBH) binaries. The two events are named GW200105_162426 and GW200115_042309, abbreviated as GW200105 and GW200115; the first was observed by LIGO Livingston and Virgo, and the second by all three LIGO-Virgo detectors. The source of GW200105 has component masses $8.9-1.5+1.2M\odot$ and $1.9-0.2+0.3M\odot$, whereas the source of GW200115 has component masses $5.7-2.1+1.8M\odot$ and $1.5-0.3+0.7M\odot$ (all measurements quoted at the 90% credible level). The probability that the secondary's mass is below the maximal mass of a neutron star is 89%-96% and 87%-98%, respectively, for GW200105 and GW200115, with the ranges arising from different astrophysical assumptions. The source luminosity distances are $280-110+110$ Mpc and $300-100+150$ Mpc, respectively. The magnitude of the primary spin of GW200105 is less than 0.23 at the 90% credible level, and its orientation is unconstrained. For GW200115, the primary spin has a negative spin projection onto the orbital angular momentum at 88% probability. We are unable to constrain spin or tidal deformation of the secondary component for either event. We infer a NSBH merger rate density of $45-33+75\mathrm{Gpc}-3\mathrm{yr}-1$ when assuming GW200105 and GW200115 are representative of the NSBH population, or $130-69+112\mathrm{Gpc}-3\mathrm{yr}-1$ under the assumption of a broader distribution of component masses.