Purpose: To determine the criterion validity of the SET-ULM (Spatial Exploration Test of Upper Limb Mobility), a functional workspace test. Materials and Methods: A prospective study from July 2017 to November 2018 in 30 children with SMA type 1 or 2. All children underwent assessment with the SET-ULM and the Motor Function Measure (MFM). Results: We included 30 children. Median (Q1; Q3) MFM D1 (standing ability, ambulation and transfers), D2 (axial and proximal motor function), D3 (distal motor function) scores, Total MFM and Total SET-ULM active score were respectively 2.6% (2.6u20133.8); 45.8% (19.9u201365.3); 57.7% (36.9u201380.9); 35.4% (16.7u201343.2) and 70.2% (49.7u201397.9). Total SET-ULM active score was strongly correlated with the MFM D2 dimension score (rho 0.82; p < 001), with the D3 dimension (rho 0.86; p < 0.001) and with the Total MFM score (rho 0.89; p < 0.005). Total SET-ULM active score differed between SMA types (p < 0.01). Conclusion: The SET-ULM has good criterion validity for the evaluation of available horizontal active upper limb workspace in children with SMA1 and SMA2. Future studies should evaluate reliability and sensitivity to change during a longitudinal follow-up study, as well as in a longitudinal trial of therapeutic effectiveness. Clinical Trials: NCT03223051IMPLICATIONS FOR REHABILITATION The Spatial Exploration Test for Upper Limb Mobility is a useful adjunct to the Motor Function Measure. It provides a precise evaluation of horizontal reaching ability. The Spatial Exploration Test for Upper Limb Mobility will be of great clinical utility for the evaluation of the effects of treatments for spinal muscular atrophy.