The invention relates to a device (1) for forming droplets in a microfluidic circuit, the device comprising a chamber (3) containing a first fluid and defined by two opposite walls (10, 11) that diverge relative to each other in at least one given direction, and a microchannel (8) containing a second fluid and leading into a zone of said chamber (3) that is upstream relative to the given direction, the outlet of the microchannel (8) into the chamber (3) constituting an enlargement in the flow section for the second fluid, and the enlargement giving rise to droplets (14) of the second fluid forming within the first fluid.