BackgroundTrade liberalisation has contributed to obesogenic food environments globally. Small Island Developing States (SIDS) have some of the world's highest rates of obesity and nutrition-related noncommunicable diseases. Nutrition regulations have been recognised as necessary population health measures for combating malnutrition, however, legally-binding trade and investment agreements (TIAs) can constrain the policy options available to governments. Geographical, economic, historical, and cultural contexts of SIDS may place them at greater risk of TIA constraints resulting in barriers to the uptake of public health nutrition policies. This article explores the perceptions and experiences of key SIDS nutrition and trade policy stakeholders regarding SIDS' ability to formulate and implement healthy nutrition policies in the context of TIAs.MethodsTwelve semi-structured interviews were conducted with key Pacific and Caribbean stakeholders. Analysis was performed via a critical realist grounded theory approach. TIA constraints to policy space, challenges faced by SIDS, and solutions for improving nutrition policy space were identified.FindingsParticipants identified that TIAs did not substantively constrain nutrition policy so long as the policy targeted a legitimate public health objective, was evidenced-based, non-discriminatory, non-arbitrary, necessary, and the least trade-restrictive measure available. However, TIAs were perceived to pose structural and procedural constraints in the form of regulatory chill, increased burden of ensuring trade-compliant nutrition policies, unfair TIA negotiation processes, inconsistent perceptions of 'unhealthy' foods, trade liberalisation ideology, and industry interference. These constraints were noted to be particularly acute for SIDS due to their financial and capacity constraints, industry influence and limited international power.ConclusionTIA obligations were deemed unlikely to substantively prevent meaningful public health nutrition policies from being developed and implemented in SIDS if nutrition policy met specific trade principles. However, concerns were noted that some of these principles may impose procedural and structural constraints that risked preventing, postponing or diluting potential nutrition policies. These constraints may be particularly problematic for SIDS due to their contextual challenges. Despite this, local, regional and international actors can increase SIDS' policy space through capacity building, fostering multisectoral collaboration, developing conflict of interest policies, improving TIA negotiation processes, and championing the prioritisation of public health nutrition in trade governance.