The main aim of this study was to evaluate the Early Grade Writing Assessment for Kindergarten (EGWA-K), which is grounded in foundational literacy skills, for its validity and diagnostic accuracy in identifying children at risk of developing early learning disabilities in writing (LDW). To the best of our knowledge, no such tool exists for Spanish-speaking children. A sample of 363 Spanish kindergarten children aged 4.7 to 6.6 years was selected for this purpose. The EGWA-K protocol, comprising four tasks (i.e., phonological awareness, writing words from pictures, free writing of words, and oral narrative), was administered to the sample. Additionally, two criterion tasks based on free letter writing and teacher ratings were included for validation purposes. First, an exploratory factor analysis (EFA) was conducted using parallel analysis to determine the dimensionality of the EGWA-K, which was confirmed by a confirmatory factor analysis (CFA) yielding two factors (one related to transcription ability and the other to narrative competence). Additionally, measurement invariance was confirmed at the configural, metric, scalar, and residual levels by gender. The EGWA-K had an acceptable area under the ROC curve (AUC in the 0.87 range), particularly when free letter writing was utilized as the criterion measure, with adequate sensitivity (0.90) and specificity (0.71) indices. Therefore, a reliable and construct-valid screening tool such as the EGWA-K is now available for the early identification of LDW in Spanish kindergarten children.