We examined the longitudinal contribution of awareness of inflections and derivations to reading comprehension in Arabic, a morphologically rich language, among 734 second graders. Morphological awareness, phonological awareness, word decoding and reading comprehension tasks were delivered at the beginning and at the end of the school year. Results indicated that readers improved in morphological awareness and reading comprehension over the course of the year. More importantly, in general, morphological awareness at the beginning of the year predicted reading comprehension at the end of the year. Moreover, inflectional awareness at the beginning of the year predicted reading comprehension at the end of the year among readers with low levels of morphological awareness, whereas both inflectional and derivational awareness predicted reading comprehension in high morphological awareness readers, beyond the contribution of word decoding. The results highlight the importance of testing awareness of inflections and derivations separately, and the differential role of inflectional and derivational awareness to Arabic reading comprehension in young readers with different levels of morphological awareness.