Background: There is no single classification system or diagnostic protocol for speech sound disorder (SSD). This makes it difficult to collect large-scale outcome data and determine which interventions work best for which subtypes of SSD. The United Kingdom is unique in that its publicly funded healthcare system allows the collection of such outcome data across large numbers of children; however, a necessary first step towards this is to agree on a consistent diagnostic protocol and classification system for SSD that is feasible for use in the UK healthcare system. Aims: This study aimed to achieve an initial clinician-led UK consensus on a diagnostic protocol and classification system for SSD of unknown origin. Methods and Procedures: A mixed methods participatory design was used. Five UK health services provided SSD paperwork such as local guidelines and protocols for content analysis. Two participatory workshops were used to agree on: (1) a classification system, (2) subtype labels and definitions, and (3) a feasible diagnostic protocol for SSD. The finalised consensus was presented to a national meeting of 283 SLTs to determine the feasibility of the protocol for clinicians across the whole of the United Kingdom. Outcomes and Results: Workshop participants agreed that the Differential Diagnostic Classification System was preferred for the United Kingdom. A minimum diagnostic protocol, with additional assessment for complex SSD, was agreed. Over 90% of the national SLT meeting agreed that they could implement the definitions and protocol. Conclusions and Implications: A preliminary diagnostic protocol, classification system, and subtype names and definitions were agreed upon and are broadly in line with those proposed by Dodd (2014). Future work will trial the consensus protocol and classification system in the United Kingdom to investigate treatment outcomes and refine the protocol.