The rapid sol–gel synthesis of macroscopic quantities of nanodiamond aerogel (NDAG) is reported at standard temperature and pressure using acid-catalyzed covalent crosslinking of air-oxidized detonation nanodiamond (DND) nanocrystals. Acetonitrile acts as a polar, aprotic solvent both to form a colloidal dispersion of DND particles and to conduct acid-catalyzed polycondensation reactions between resorcinol and formaldehyde (RF) small molecule precursors. Several characterization techniques show that nanodiamond grains are connected via covalent bonding with RF molecules to form a porous, three-dimensional network. Solvent exchange into liquid carbon dioxide and subsequent supercritical drying of NDAGs are used to recover low-density (151 mg/cm3), three-dimensional monolithic aerogels that exhibit surface areas in excess of 589 m2/g. The large accessible pore volume from the rapidly synthesized, macroscopic NDAG materials suggests a range of potential applications in catalysis, sensing, energy storage, as well as inert excipients for small-molecule pharmaceuticals.