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Abstract Using published simulations of the 10 yr Legacy Survey of Space and Time (LSST), we forecast its ability to determine the masses of individual main-belt asteroids (MBAs) through precise astrometry of any pairs of the ≈1.2 million known MBAs undergoing close gravitational encounters during the survey. The uncertaintyσ_Ion the impulse applied to a tracer asteroid by its deflector is derived from the Fisher matrix of the tracer’s astrometric data, including an azimuthal acceleration from the Yarkovsky effect as a free parameter for each tracer. If only LSST observations are available, the mean forecastedσ_Iis 7 × 10⁻⁶m s⁻¹for MBAs at apparent magnitudem_V < 19.5, degrading ≈10× form_V = 23. The forecasted median uncertainty on the mass of a deflector MBA is ≈3 × 10⁻¹⁴M_⊙, with a wide range of variation depending on the (random) configurations of the closest encounters. All of these figures improve ≈ fivefold if strong pre-LSST astrometry is available. Use of LSST data will increase the number of MBAs with masses measured to 20% accuracy or better from the present ≈70 to between 200 and 550, depending on quality of pre-LSST data, if all MBAs have mass-to-reflected-light ratios (M/Ls) at the low end of the currently measured bodies. If high-end M/Ls are more common, 580–1100 bodies will attain a signal-to-noise ratio > 5, including nearly all bodies with absolute magnitudeH < 10 and some as faint asH = 13. We forecast that ≈20 new Jupiter Trojans could also have masses measured at <20% accuracy. Tables of the measurable deflector MBAs and their tracers are provided.