AbstractWe report the first detection of a short-duration (<3 months) outburst of an H₂CO 6cm maser based on multi-epoch observations of IRAS18566+0408 obtained with Arecibo, the Green Bank Telescope, and the Very Large Array. The H₂CO maser was observed nine times between 2002 and 2005. In May 2006 we began a two-year program of monthly monitoring with the Arecibo Telescope. The H₂CO maser in IRAS18566+0408 is coincident with a young massive stellar object and the line profile of the maser suggests that the H₂CO emission originates from two different regions, perhaps associated with the kinematics of a circumstellar disk.