Biogenic isoprene plays an important role in tropospheric chemistry. Current isoprene emission estimates are highly uncertain because of a lack of direct observations. Formaldehyde (HCHO) is a high-yield product of isoprene oxidation. The short photochemical lifetime of HCHO allows the observation of this trace gas to help constrain isoprene emissions. We use HCHO column observations from the Global Ozone Monitoring Experiment (GOME). These global data are particularly useful for studying large isoprene emissions from the tropics, where in situ observations are sparse. Using the global Goddard Earth Observing System–Chemistry (GEOS-CHEM) chemical transport model as the forward model, a Bayesian inversion of GOME HCHO observations from September 1996 to August 1997 is conducted to calculate global isoprene emissions. Column contributions to HCHO from 10 biogenic sources, in addition to biomass-burning and industrial sources, are considered. The inversion of these 12 HCHO sources is conducted separately for eight geographical regions (North America, Europe, east Asia, India, Southeast Asia, South America, Africa, and Australia). GOME measurements with high signal-to-noise ratios are used. The a priori simulation greatly underestimates global HCHO columns over the eight geographical regions (bias, −14 to −46%; R = 0.52–0.84). The a posteriori solution shows generally higher isoprene and biomass-burning emissions, and these emissions reduce the model biases for all regions (bias, −3.6 to −25%; R = 0.56–0.84). The negative bias in the a posteriori estimate reflects in part the uncertainty in GOME measurements. The a posteriori estimate of the annual global isoprene emissions of 566 Tg C yr−1 is ∼50% larger than the a priori estimate. This increase of global isoprene emissions significantly affects tropospheric chemistry, decreasing the global mean OH concentration by 10.8% to 0.95 × 106 molecules/cm3. The atmospheric lifetime of CH3CCl3 increases from 5.2 to 5.7 years.