Polycrystalline compacts include non-catalytic, non-carbide-forming particles in interstitial spaces between interbonded grains of hard material in a polycrystalline hard material. Cutting elements and earth-boring tools include such polycrystalline compacts. Methods of forming polycrystalline compacts include forming a polycrystalline material including a hard material and a plurality of particles comprising a non-catalytic, non-carbide-forming material. Methods of forming cutting elements include infiltrating interstitial spaces between interbonded grains of hard material in a polycrystalline material with a plurality of non-catalytic, non-carbide-forming particles.