The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft is the third NASA New Frontiers Program mission and arrived at the near-Earth asteroid (101955) Bennu in December 2018. After a thorough proximity operations phase to characterize the asteroid, the OSIRIS-REx spacecraft will fly a touch-and-go (TAG) trajectory to the asteroid’s surface to collect at least 60 g of pristine regolith sample for Earth return. To verify the success of the TAG event, the spacecraft performs an inflight measurement of the collected sample mass. This paper presents the inflight validation and characterization of the Sample Mass Measurement (SMM) technique performed during the outbound cruise and approach phases of the mission. The OSIRIS-REx spacecraft performs several slew maneuvers to utilize an in-flight measurement of spacecraft inertia based on the principle of conservation of momentum. The resulting telemetry are processed to isolate the inertia contribution due to the collected regolith sample mass. Incremental exercises allowed characterization of smaller groupings of error sources prior to validating the full method. Results from the in-flight validation activities are presented along with improvements made to the technique to minimize sources of error.