Although many waterbirds breed in colonies, documented colonisation events are rare. I studied the establishment and growth of a new breeding colony of the globally Near Threatened Dalmatian Pelican Pelecanus crispus at the newly created Karla Reservoir, Central Greece, between 2012 and 2017. The reservoir began filling with water in 2009. The first pelicans were observed in 2009 and numbers increased to 950 in June 2015. Since then the occurrence pattern changed and numbers fluctuated seasonally, being high during breeding and almost zero during summer and autumn. The first pair bred in 2011 and breeding pairs reached 445 in 2017, becoming the second largest colony of Dalmatian Pelican in Europe and manifesting an annual growth rate of 176.4%. The high rate of increase and re-sightings of ringed and tagged Dalmatian Pelicans suggest that colonisers must have been immigrants from the nearest two colonies in Kerkini Reservoir and/or Lake Prespa (northern Greece). Breeding success ranged from 0.54 to 0.88 young/nest, which is low for the species. Both the change in occurrence pattern and the low breeding success must be attributed to reasons other than nesting conditions. From a conservation point of view this new breeding colony may act as an ecological trap for Dalmatian Pelicans, as low water quality will probably continue favouring conditions leading to mass pelican mortality.