With very scanty information on invasive alien species (IAS) in Nigeria, this study was conducted to identify the invasive species in the field gene bank of the National Centre for Genetic Resources and Biotechnology (NACGRAB), Ibadan, Nigeria, on the latitude 7° 22′ north of the equator and longitude 3° 50′ east of the Greenwich Meridian, and also to assess the level of the species' disturbance to the conserved native plants in the protected area. This study involved sampling of the field gene bank and an on-site assessment and identification of the species and their effects on the indigenous plants established in the field for conservation. Twenty-five invasive plant species were identified, across 16 plant families, of which 14 were herbs, followed by vines, shrubs, and trees, all of which are presently estimated to occupy about 18% of the gene bank. The effects of IAS on the indigenous plants conserved in the field gene bank range from competition for space and nutrients and alteration of the tree canopies (thereby affecting the microclimatic conditions in the lower strata) to obstruction of the plants' reception of sunlight (which could thereby reduce the potential yield of the fruit trees in the gene bank. The paper also discusses the effects of IAS on biodiversity conservation in Nigeria. Human disturbance was observed as the major factor responsible for the spread of these IAS in the gene bank. The paper concludes by advocating stricter screening measures before introducing new plants into the country, capacity building on the early detection and management of IAS in protected areas for the technical staff, biological control, and exchange of technical information among concerned countries.