This study presents a phonological analysis of the tonal system of Opobo-Igbo, an under-documented variety within the Igbo dialect continuum. Although Igbo is widely described as a two-tone language with downstep, existing analyses are largely based on Standard Igbo and a limited number of central dialects. This paper addresses this descriptive and theoretical gap by examining the tonal behaviour of Opobo-Igbo and assessing its implications for Igbo tonal typology. Using primary data elicited from native speakers, the study adopts the framework of Autosegmental Phonology to analyse lexical and grammatical tone patterns. The findings confirm a binary High–Low tonal contrast but reveal systematic patterns of downstep and tonal alternation that are not always predictable from surface configurations alone. Tone is shown to function as a crucial marker of lexical contrast and grammatical distinction, particularly in nominal constructions and verbal paradigms. The analysis argues that downstep in Opobo-Igbo is best accounted for through floating tonal representations rather than purely phonetic lowering. These findings contribute to a more nuanced understanding of tonal interaction in Igbo and demonstrate that dialect-specific evidence is essential for refining generalisations about Igbo tone. Beyond phonological description, the study highlights the role of tone in dialect identity and linguistic differentiation, thereby contributing to both Igbo dialectology and African tone theory.