The recent developments of social networking sites (SNSs), along with the increasing usage of online shopping, has led to the emergence of social commerce platforms. Social commerce (s-commerce) is the use of Web 2.0 technologies and social media to deliver e-commerce services for consumers. The Kingdom of Saudi Arabia (KSA) has been witnessing a rapid growth in s-commerce usage, with Instagram being the most popular networks in the region. This paper is one of the few that investigates the factors affecting consumers' trust and purchase intentions in Instagram as a s-commerce platform in Saudi Arabia. The proposed model explores a number of factors, such as Social Media Influencers (SMIs), Key Opinion Leaders (KOLs) and consumer feedback, in terms of their influence on consumers' trust and purchase decisions. In addition to the effect of Maroof, which is an e-service provided by the Saudi Ministry of Commerce and Investment to evaluate the reliability of online stores. Following a quantitative approach and using Partial Least Squares Modeling (PLS-SEM), findings of this study revealed a positive relationship between consumers' trust and their purchase intentions. Additionally, the impact of SMIs and consumer feedback was shown to increase consumers' trust, in turn affecting intent to buy from Instagram stores, while the effect of Maroof and KOLs was shown to directly influence consumers' purchase intentions.