Little research has been conducted on Arabic letter knowledge. This study investigates the nature of Arabic letter knowledge, its dimensionality, and the relative difficulty of letter knowledge items, all within an item response theory (IRT) framework. Three letter knowledge tests were administered to 142 native Arabic-speaking kindergarteners (mean age = 67 months). The letter recognition task was found to be multidimensional, containing two factors, whereas the allograph and syllable tasks were found to be unidimensional. Results showed that a two parameter model fit best for all three tasks, demonstrating that items varied in degree of difficulty and in discrimination. Findings provide a subset of letters that are most useful for quickly and precisely assessing children's letter knowledge. Results are discussed in the context of orthographic and linguistic features of Arabic. Implications for assessment and instruction are discussed.