Bristlecone pine (Pinus longaeva) (PILO) trees exhibit exceptional longevity. Their tree-ring width (TRW) series offer valuable insights into climatic variability. Maximum latewood density (MXD) typically correlates better with temperature variations than TRW, yet PILO MXD records are non-existent due to methodological challenges related to their tree-ring structure. Here, we used an X-ray Computed Tomography (X-ray CT) toolchain on 51 PILO cores from the California White Mountains to build a chronology that correlates significantly (r=0.66, p<0.01) with warm-season (March-September) temperature over a large spatial extent. This led to the first X-ray CT-based temperature reconstruction (1625 – 2005 CE). Good reconstruction skill (RE=0.51, CE=0.32) shows that extending MXD records across the full length of the PILO archive could yield a robust warm-season temperature proxy for the American Southwest over millennia. This breakthrough opens avenues for measuring MXD in other challenging conifers, increasing our understanding of past climate further, particularly in lower latitudes.