The first observations of the Monarch butterfly (Danaus plexippus) in Iberia date from 1886, although breeding records emerged almost a century later: 1960 in Spain, 1980s–1990s in Madeira and Azores, and 2003 in mainland Portugal. We reviewed the history of the colonisation of mainland and insular Portugal by the Monarch butterfly and its hostplants (Gomphocarpus fruticosus, G. physocarpus and Asclepias curassavica). We also compiled available historical and recent occurrence records as a basis for countrywide surveys of the butterfly and hostplants, to update their current distributions in Portugal. Locations for only a few of the older records represented newly rediscovered populations in the field, although recent occurrences were often confirmed. Hostplants were scarce and monarchs absent in northern and central mainland Portugal, but both were quite common in the southwest. In Madeira, hostplants were found in two locations, while monarchs were common and widespread. In the Azores, small hostplant patches were observed on four of seven surveyed islands, whereas monarchs were rare and restricted to two islands. Abandoned/semi-abandoned orange orchards represent the butterfly’s stronghold in Portugal, with the species being increasingly scarce along rivers and road verges where hostplants are declining. Hostplant persistence is unstable, with many patches removed, while others have expanded or colonised new areas. Overall, hostplants appear to be declining, with implications for the persistence of monarch butterflies in the country.