Purpose: This study examined the differences in spoken Arabic (SpA) and standard Arabic (StA) in inflectional (gender, number, possessive pronouns, and tense) construction use in Arabic among preschoolers. Moreover, we tested the contribution of the inflectional constructions possessed in kindergarten to reading skills in the first grade and examined whether this morphological contribution differs between SpA and StA. Method: We assessed 261 Arabic-speaking kindergartners for 1 year until the end of first grade for inflectional knowledge in kindergarten and reading skills in first grade (reading accuracy and fluency, spelling, and reading comprehension). Results: The findings revealed that among inflections, prevalence of performance on gender constructions was the highest, followed by number and possessive pronouns, and lowest performance for tense constructions. Although the performance for SpA was higher than for StA in all constructions, similar patterns were observed except similarity between gender and number in StA. Moreover, the results indicate a significant contribution of almost all inflectional constructions (except possessive pronouns) possessed in kindergarten to all reading skills in the first grade. However, tense did not contribute to reading comprehension, and possessive pronouns did not contribute to any of the reading measures. Regarding diglossia, although the claims that linguistic components in StA are not represented in the mental lexicon, StA accounted for an additional significant 2%-3% of the explained variance in Step 2 (which checked the practical significance of statistically significant results) in all reading measures. Conclusions: This study highlights the impact of diglossia-specific morphological differences (prevalence of the use of the morphological construction in Arabic in SpA vs. StA) on reading and literacy measures, especially the contribution of morphological awareness in SpA, which may provide a stronger basis for StA reading skills. The implications of these results are discussed, especially regarding exposing children to the morphological representations of both the SpA and StA forms to promote reading and literacy in Arabic.