Background: Children with speech sound disorders (SSD) are at higher risk of communication breakdown, but the impact of having an SSD may vary from child to child. Determining the severity of SSD helps speech-language therapists (SLTs) to recognise the extent of the problem and to identify and prioritise children who require intervention. Aims: This study aimed to identify severity factors for SSD in order to develop a multifactorial Speech Sound Disorder Severity Construct (SSDSC) using SLTs' views and the International Classification of Functioning, Disability and Health (ICF). Method: In an explorative five-staged qualitative study, the research question was answered: 'How do SLTs determine the severity of SSD in children?'. A total of 91 SLTs from The Netherlands participated in data collection and analysis. The iterative process included three different qualitative research methodologies (thematic analysis [TA], constructivist grounded theory [CGT] and content analysis [CA]) to ensure validation of the results by means of method triangulation. Results: SLTs considered nine themes: intelligibility, speech accuracy, persistence, the child's perception, impact, communicative participation, concomitant factors, professional point of view, and environmental factors. The themes were summarised in three main severity factors: (I) Speech accuracy, (II) The child's perception of the impact of their speech, and (III) Intelligibility in communication. Other severity factors were concomitant factors and impact. Expertise and support were identified as facilitators or barriers that may worsen or relieve the severity of SSD. Conclusions: This study highlights the need for SLTs to rethink how they think about severity as a simplistic construct reflecting only speech accuracy. It is recommended that a broader holistic approach to measuring severity is adopted.