Characterization of Earth’s magnetic field is key to understanding the dynamics of core flows and the dynamo. Satellite measurements of the magnetic field normally use precise magnetometers on a few spacecraft to acquire data over the entire globe over periods of months to years. The advent of commercial satellite constellations of tens to hundreds of satellites may offer complementary observations, even with low-precision magnetometers, providing rapid global coverage. Here we assess whether the magnetic field data from the Iridium Communications constellation of 66 low Earth orbiting satellites can be used to determine the geometry of Earth’s main field. The Iridium satellites are in near polar, 86° inclination, 780 km altitude, circular orbits, with 11 satellites in each of six orbit planes evenly spaced in longitude. We use data from the first-generation Iridium satellites, launched in the late 1990s, and acquired for scientific analysis beginning in January 2010. Although digitized with 30 nT resolution, the uncertainties in the data are random errors so that the statistics of 300,000 samples/day allow determination of the average magnetic field in 9° latitude by 9° longitude bins to about 3 nT. The data reduction, inter-calibration, quiet interval selection, and uncertainty assessment are described. Time series of spherical harmonic coefficients are used to identify artifacts and derive maps of corrected residuals at the average Iridium orbit altitude. From 2010 to 2015 the evolution of the field agrees on average between Iridium and the CHAOS 7.4 model to within 30 nT standard deviation, or ~5 nT/yr.