Research into geographic variation in the agamid lizard Ctenophorus fordi supports a pattern of regional lineage diversity identified in earlier genetic studies, and provides new information on differences in morphology diagnostic of these lineages. One of the most genetically divergent and morphologically distinctive of these lineages is that consisting of populations from Triodia Mallee habitat of the red sandplains of inland southeast Australia. Populations from this region are the sister lineage to all other “C. fordi”, a suite of four genetically regionally discrete lineages distributed across the arid inland of southern Australia. They can be distinguished by a unique black “T” shaped chest pattern in adult males which, in combination with certain features of body proportions and scalation, diagnose the lineage from all other “C. fordi”. On the strength of these differences we describe populations belonging to this lineage as a new species, Ctenophorus spinodomus sp. nov. This new species is highly restricted in the habitat it occupies, and its ecology tied to a reliance on the presence of Triodia hummock grass groundcover (spinifex) for shelter, foraging and social interactions. It appears to be most abundant in areas of extensive and healthy Triodia that develop 20–50 years post-burn, and as a consequence too many or too few fires can both have negative impacts on the suitability of hummock grass groundcover for this species. Mallee habitat in southeast Australia is fragmented, and large fires in the smaller isolated areas of habitat could result in loss of suitable habitat for the species, resulting in localized extinction with no opportunity for recruitment. These factors in combination with ongoing loss of habitat place the remaining populations of C. spinodomus sp. nov. at a high level of vulnerability.