Land speculation has been an integral component of the political economy of land development in American urban history. In the American Sunbelt, land speculation occurs amid progrowth governance regimes that engage in intermunicipal competition for capital investment. This article presents a mixed-methods case study of vacant land speculation in downtown Phoenix, Arizona, before, during, and after the mid-2000s property boom. Results indicate that land speculation represented a significant barrier to both public and private infill development efforts, and that some municipal development initiatives actually facilitated private speculative profits. Speculative strategies are enabled when weaknesses in the coordination and bargaining power of urban growth regimes, derived from conflict within and between governmental scales, can be exploited by individual market actors. The self-propulsive nature of speculative property market cycles, unconstrained by local regimes increasingly dependent on nonlocal capital investment, represents an autonomous force actively orienting entries into property markets and influencing the ability to enact sustainable infill development.