Arab students who learn English as a foreign language, especially Saudi students, face different challenges during the process of learning of the four English language skills, especially writing and its component (spelling). This paper aims to investigate the preceded causes of students' spelling errors. The main research question sought to be answered is: What are the causes of spelling errors made by the Saudi university students? The research participants were 15 students in the English Language Department at Tabuk University and 15 English language lecturers from the same department. Group structured interviews were designed for the lecturers and students. The findings reveal that there are different causes of students' spelling errors such as the education system and university syllabus, students' learning attitude, and the interference between English and Arabic language. This paper concludes that the spelling errors which Saudi university students commit were caused by the negative impacts of their education system and syllabus, where the syllabus ignores the importance of spelling rules and techniques, and the interference between English and Arabic language when the learners refer to their mother tongue while writing in the English language. It is hoped that the findings revealed in this study will help the policymakers in taking necessary actions in improving the learning experience of Arab learners of English. This paper calls for a reform in the English language teaching in Saudi education system so that spelling is given the required emphasis as it is the foundation of English proficiency.