Investing resources into protecting small, isolated tropical forest fragments can require considerable justification, especially in the context of limited resources and competing development pressures. Nevertheless, fragments may be suitable targets for conservation if they offer irreplaceable biodiversity outcomes and align with conservation aspirations. This is particularly true for island systems such as Oceania, where tropical lowland forest fragments may be the last remaining representatives of critical habitat due to agriculture or urban expansion on limited flat land. Despite this urgency, the decision to invest significant resources into a permanent conservation investment should be informed by a systematic framework to evaluate relevant conservation criteria for one or multiple sites. Here we present a case study where we applied the criteria-based framework of the Forest Legacy Program of the United States Forest Service, to assess the importance, threat, and strategic benefits of the last known remnant (9.33 ha) of lowland lava flow forest in American Samoa (Polynesia). We expounded on the FLP framework by providing examples of potential evidence and corresponding datasets that could inform the conservation criteria. For the case study, we conducted a comprehensive high-precision tree census to quantify forest structure and composition and compare with other forest types in the country, which revealed high conservation importance based on unique community composition and irreplaceable tree species representations, including Pometia pinnata, Dendrocnide harveyi, and two banyan figs Ficus obliqua and F. prolixa. Analysis of historical (70-year) aerial imagery and recent land cover maps, evidence of recent small-scale incursions, documented attempts by the landowners to obtain development permits, and the need for a court-imposed injunction and a conservation lease to prevent further encroachment, all indicated a high likelihood of conversion (threat) if left unprotected. Finally, we found that protection of this fragment would strategically align with the conservation, management, and education portfolios of the American Samoa government, and provide significant cultural and well-being benefits to the public, specific to the fact that this is the last representative of lava flow forest and is easily accessible to the public. We conclude that there is strong justification for conservation of Naumati forest, which in this case would be through a land purchase. More broadly, criteria-based approaches are often broad and flexible and can be widely applied, but they require significant work to collect data and generate evidence that is meaningful to conservation practitioners working on-the-ground. Effectively linking data to evidence to criteria and ultimately to decision-making is key to justifying sound conservation investments.