A nutrition transition characterized by dietary shifts from traditional diets to processed foods is underway in many low and middle income countries (LMICs). Foods that are high in fat, sugar and salt (HFSS) now make up the majority of advertised foods and are frequently promoted through a variety of social media channels. This marketing is primarily aimed at adolescents due to their increasing autonomy, spending power, and significant use of media devices. Consequently, the evidence details their heavy exposure to the marketing of HFSS foods particularly on social media, raising concerns related to consequences such as fuelling the childhood obesity epidemic currently being experienced in LMICs including Nigeria. Given that overweight and obesity are among the major risk factors for non-communicable diseases and countries like Nigeria are already experiencing a double burden of disease, there is an urgent need for more research and policy action to protect children living in these countries from exposure to unhealthy food marketing. However, public health efforts to address this issue have been constrained by the limited and disproportionate nature of the evidence on adolescents' exposure to unhealthy food marketing and its influence on their food choices. This research program aims to address these gaps and inform policy and health promoting actions by examining how transnational fast-food marketing influences the purchase intentions of Nigerian adolescents. This thesis incorporates three interrelated research studies, each building on one another to address the overall research aim. The findings have been published or are under review in peer-reviewed journals. This research is underpinned by the pragmatic paradigm and used a mixed methods approach where the researcher collects, analyses, and integrates both quantitative and qualitative data in a single research program or in multiple studies in a sustained program of inquiry. A conceptual framework was developed to guide the research. Study 1 was a systematic review that identified the strategies used by global fast-food brands to promote their products to adolescents in LMICs. 12 articles met the inclusion criteria and were critically appraised. The findings of this review identified 15 adolescent-targeted strategies used across LMICs and highlighted the prominent use of social media as an important marketing channel to reach adolescents in those settings. The findings of this review have been published in a peer-review academic journal and informed the development of Study 2. Study 2 included a content analysis of the Instagram accounts of global fast-food brands in Nigeria. A total of 576 posts were analysed, using a codebook developed based on the relevant literature, to identify adolescent-targeted strategies. The results identified the prominent use of emotional appeal, product appeal, premium language and teen language. It also found that the utilisation of product appeal was positively associated with user engagement. The findings of this study have been accepted for publication in a peer-review academic journal and informed the development of Study 3. Study 3 was a cross-sectional qualitative study involving semi-structured group interviews with 15 Nigerian adolescents to explore how they are influenced by transnational fast-food marketing. The findings revealed that adolescents perceived that apart from being tasty, fast-food brought people together and offered them value for money. These perceptions led to positive purchase intentions towards fast foods. Attributes of the promotional posts which led to the influential perceptions derived were also identified. This knowledge has tremendous potential to inform the effective promotion of healthy foods to encourage adolescents to make healthier choices. This study has been written up in a manuscript format and submitted to a peer-reviewed academic journal. Collectively the findings of this research provide a holistic understanding of the nature, extent and influence of Nigerian adolescents' exposure to fast-food marketing. To date, no research has examined the influence of social media based fast-food marketing on adolescents living in sub-Saharan Africa. Global evidence on the influence of fast-food marketing from an adolescent's perspective is also limited; a knowledge gap that is a major barrier to the design of effective healthy eating campaigns to counter the effect of unhealthy food marketing. This research identified attributes of fast-food marketing which influence adolescents' food choices, and how they are operationalised. Thus, insights from the studies conducted within this program could inform policy actions to moderate unhealthy food marketing and initiate the design of healthy eating interventions to counter such marketing within similar contexts. By identifying new, context-specific pathways between promotion and effect, this research also lays the foundation for researchers to explore opportunities for contemporary food marketing techniques to inform healthier food choices in low-income settings.