The study aimed to investigate the transformative effect of overt instruction on the acquisition of irregular inflectional morphemes by Saudi EFL learners. A mixed-method approach consisting of quantitative data and complemented with a qualitative descriptive analysis was adopted for this study to measure the transformative effect of explicit instruction through pretest and posttest procedures. Input Processing Theory and Processability Theory were used in this study to see the transformative effect of overt instruction between the control and experimental groups. The study employed visual aids and metalinguistic discussion instructional techniques to treat the experimental group with a one-hour session once a week, other than the normal classroom instruction sessions. In total, it continued for seven consecutive weeks. The results demonstrated that a p-value less than 0.001 for all three hypotheses developed in the study posited in favour of the alternative hypothesis and rejected the null hypothesis. Hence, the two groups significantly differed in their vocabulary knowledge: morpheme recognition and morpheme use in writing. The results support the hypotheses developed initially in this study that overt instruction of inflectional irregular morphological knowledge transfers to the EFL learners’ writing skills and facilitates L2 acquisition. The study has significant implications for foreign language learners, L2 teachers, and researchers.