Numerous studies on Arabic linguistics make reference to the phonological word (PW) as a constituent that serves as the domain for various phonological phenomena, however, there is no clear and precise definition in the Arabic linguistics literature of what actually constitutes as a PW and whether it consists of just the bare morphological stem or whether it includes affixes as well. The purpose of this paper is to gain a better understanding of the status of this phonological constituent in Najdi Arabic (NA), a variety of Arabic spoken in Najd, located in the central region of Saudi Arabia. Affixation and other phonological processes are thoroughly investigated to examine how they interact with the stem in an attempt to give an accurate and precise definition of the domain of the PW. Using syllabification and stress placement as reliable diagnostic tools, in addition to other phonological processes, it is concluded that the PW in NA consists of the morphological stem including all affixes (i.e. prefixes and suffixes, both inflectional and derivational) and functions words (e.g., prepositions). Evidence that supports this conclusion stems from the fact that the stem plus affixes comprise the domain for syllabification and the stress assignment rules as well.