We report the detection of numerous absorption lines of the nu₁ band of gaseous formaldehyde (H2CO) near 3.6 mum towards the high-mass protostar W33A. This is the first infrared detection of gaseous H2CO in an interstellar cloud. An upper limit toward RAFGL 7009S is also reported. The column density of H2CO detected at 3.6 mum toward W33A is much higher than that measured in large beam millimeter wave observations toward the same source, suggesting that it is concentrated close to the infrared continuum source. The mean temperature of the H2CO, derived from LTE spectral modelling, is ~100 K, which is close to the values found for most other gas phase species observed in absorption and is the expected temperature of the cloud at the dust mantle sublimation interface, implying a large jump in the abundance of gaseous H2CO from the surrounding cold cloud to the warm gas. The ratio of gaseous and solid H2CO column densities is about 0.03 for W33A and less than 0.02 for RAFGL 7009S. We discuss this detection in the context of the two most likely mechanisms for H2CO formation: grain mantle evaporation, and gas phase reactions between species produced by photodissociation of grain mantle evaporants.