In the Netherlands, children (2;0–5;0) with (presumed) severe DLD attend special treatment groups. In these groups, speech sound disorders (SSDs) are usually treated in individual therapy. The aim of this study is to examine the efficacy of an added phonological group intervention. Parallel to individual therapy, these children received a group intervention from trained childcare professionals. A within-subject multiple-baseline design was chosen to compare children’s development before and after the intervention. Children tested at all six time points were included in the intervention study ( n = 31, age range: 28–46 months). Of the children attending the selected groups, 77% have an SSD. The participating children demonstrate with a severe SSD based on their caregivers’ report of their intelligibility at baseline. Their phonological production skills increased from a mean Percentage Consonants Correct (PCC) of 42–57% after the intervention compared to no significant increase during the measurements before ...