Goethite is a common iron oxyhydroxide and an important mineral in the biogeochemical cycle of iron at the Earth's surface. However, environmental controls on its formation and alteration have been difficult to quantify. Perhaps this is because past studies have traditionally used needle-shaped goethite particles that are substantially larger than the goethite nanoparticles commonly observed in nature. High-resolution transmission electron microscopy and low-temperature magnetometry of synthetic goethite nanocrystals show that when aqueous suspensions are aged at 90°C, the nanocrystals grow almost exclusively by oriented aggregation of 3–4 nm primary nanocrystals. These primary particles are superparamagnetic above about 35 K, as shown by variations of magnetic susceptibility with temperature. At low temperature (<300 K), both remanent and induced magnetizations of primary nanoparticles are about an order of magnitude larger than for micron-sized goethite and can act as magnetic signatures of nanophase in controlled environments.