Invasive alien plants have a long history of establishment in the national parks of South Africa. In particular, Opuntia stricta (sour prickly pear) has invaded several protected areas in the country, threatening the biodiversity conservation mandate of these conservation areas. This article focuses on the economic estimation of O. stricta's negative impacts in protected areas by using Contingent Valuation surveys conducted amongst a sample of tourists in the Pilanesberg National Park (North West Parks and Tourism Board, South Africa). Tourists' familiarity and awareness of selected invasive alien plants and their willingness to pay for the implementation of a control programme for O. stricta were assessed. The results show that many tourists are familiar with invasive alien plants and their (positive and negative) impacts and, in particular, perceived the presence of O. stricta to be negative, due to the impacts on aesthetics and recreation. Socio-demographic characteristics, as well as individual attitudes and biocentric beliefs, have an influence on the willingness to contribute financially to a control programme for O. stricta. The individual willingness to pay assessment found that the majority of respondents (78%) were willing to pay a higher entrance fee (an additional R57.30 or $7.00 per day) for a hypothetical programme to control the invasion of O. stricta in the Pilanesberg National Park. The willingness of tourists to pay for O. stricta management provides useful insights in the decision-making process of park management. The results are encouraging, since, in general, tourists are aware of the problem and are in support of providing additional economic input for preventing future alien plant invasions.