Semantic annotation is the process of adding semantic metadata to resources. Semantic metadata is data concerning the meaning of entities and the relationships that exist. Semantic annotation cannot be performed without an ontology suitable for the task. In this research paper, we describe the design, implementation, and evaluation of a lexical ontology for Arabic semantic relations. The main purpose of the ontology is to facilitate the task of semantic annotation of the Arabic textual content. The ontology was evaluated for usability and usefulness using a prototype system for the automated semantic annotation of Arabic text. The results of the evaluation indicated that the ontology was fit for the purpose of semantic annotation of Arabic text with lexical relations. The evaluation has also revealed important findings and recommendations for designing Arabic semantic annotation tools.