Audit report lag is considered to be an indicator of audit punctuality and efficiency, while the existence of factors that may determine this lag has been signalled in previous studies. The objective of this study is to identify the determinants of audit report lag in Spanish SMEs, and to analyse the potential singularities of these factors with regards to non-SMEs. With this aim in mind, a set of factors has been analysed; in the previous literature, a relationship has been observed between these factors and the characteristics of the audited company, the auditor and the audit exercise. Additionally, the possible influence of financial crisis and recovery periods is analysed. The sample analysed is made up of 3217 unlisted Spanish firms for the years 2008–2015. Multiple linear regression analysis is used, modelling the audit report lag based on the independent variables related to the characteristics of the audited company, the auditor and the audit exercise. Based on the results obtained, we can support the existence of differences among the independent variables responsible for the audit report lag based on company type (SME/non-SME), with a larger number of factors impacting the audit report lag in SMEs. It is worth noting the significant relationship between audit report lag and opinion and crisis variables, both for the SME and non-SME models. In the case of SMEs, links between audit report lag and the likelihood of bankruptcy, auditor type, number of economic activities carried out by the audited company, the industry to which the audited company pertains, and audit fees were also observed. Furthermore, we can conclude that audit report lag is greater in SMEs and that the independent variables explaining report lag differ according to whether the company is an SME or not.