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)(+1.2) and 1.9(-0.2)(+0.3)M(circle dot), whereas the source of GW200115 has component masses 5.7(-2.1)(+1.8) and 1.5(-0.3)(+0.7)M(circle dot) (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) 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 the spin or tidal deformation of the secondary component for either event. We infer an NSBH merger rate density of 45-(+75)(33) Gpc(-3) yr(-1) when assuming that GW200105 and GW200115 are representative of the NSBH population or 130(-69)(+112) Gpc(-3) yr(-1) under the assumption of a broader distribution of component masses.