This article investigates how the depth of open innovation (OI) practices influences marketing performance in the small and medium scale enterprises (SME) sector. We adopted the quantitative survey approach, and we sampled and elicited data from 72 senior staff from the UK’s SMEs sector using a survey questionnaire. Thereafter, the collected primary data were analysed using SPSS version 21. Findings show that (a) the depth of OI has a curvilinear relationship with radical and incremental product performance (PP) and (b) radical innovation has a moderating influence on the depth of OI and marketing performance of SME products. Hence, the implementation OI strategies would support radical PP of SMEs. Recommendations are made for SMEs’ policymakers to adopt and maintain a deeper relationship with chosen innovation partners in the SMEs product improvement process for the purpose of realising mutually supportive innovations, especially in radically transforming PP.