A polycrystalline diamond compact (PDC) has a diamond matrix including inter-bonded diamond grains and nanodiamond agglomerates within interstitial spaces in the diamond matrix. A volume percentage of the nanodiamond agglomerates in the PDC may be greater than or equal to a percolation threshold volume of the nanodiamond agglomerates in the PDC, and a remainder of the volume of the PDC may be at least substantially comprised by the diamond matrix. The PDC may be at least substantially free of metal solvent catalyst material. Earth-boring tools include one or more such PDCs. A method of manufacturing a PDC includes mixing diamond grains with nanodiamond agglomerates to form a mixture, and subjecting the mixture to a high-temperature/high-pressure (HTHP) sintering process to form the PDC without any substantial assistance from a metal solvent catalyst material.