The coronavirus disease 2019 (COVID-19) pandemic has challenged public health and behaviour change programmes, and has led to the development of innovative interventions and research. In low -and middle-income countries (LMICs) such as Nigeria, new strategies to promote vaccination, increase pro-vaccination social norms, and reduce vaccine hesitancy have been deployed through social media campaigns and evaluated using digital media platforms. We conducted two randomised experimental evaluations of social media content designed to promote COVID-19 vaccination and to complement research on a nationwide vaccination promotion campaign in Nigeria run in 2022. We conducted two studies in March and August 2022 among Nigerians drawn from 31 states that had not been targeted in the aforementioned nationwide campaign. We randomised the participants to either receive the pro-vaccination social media campaign or not and collected data at pre- and post-test time points to evaluate psychosocial predictors of vaccination and vaccination outcomes following the Theory of Change based on Diffusion of Innovations; the Social Norms Theory, and the Motivation, Opportunity, Ability (MOA) framework. Data were collected through a novel intervention delivery and data collection platform through social media. We found that pro-vaccination social norms and vaccination rates increased, while vaccine hesitancy decreased among participants randomised to the social media intervention study arm. Social media campaigns are a promising approach to increasing vaccination at scale in LMICs, while social norms are an important factor in promoting vaccination, which is consistent with the Social Norms Theory. This study demonstrates the capability and potential of new social media-based data collection techniques. We describe implications for future vaccination campaigns and identify future research priorities in this area. Pan African Clinical Trial Registry: PACTR202310811597445.