Crustose coralline algae (CCA) are globally distributed calcifying macroalgae that can grow as free-living rhodoliths, act as ecosystem engineers by creating complex three-dimensional habitats, and contribute to the carbon cycle. The dimensions of cells in the carbonate skeleton influence rhodolith structural integrity and, consequently, the ecological functions these rhodoliths provide, yet CCA cellular responses to environmental change remain poorly resolved. This study quantifies multi-decadal variability in skeletal cell dimensions of the rhodolith-forming CCA <em>Sporolithon nodosum </em>from New Zealand and evaluates the relative importance of sea surface radiation (SSR), sea surface temperature (SST) and sea surface CO₂ partial pressure (<em>p</em>CO₂) as potential drivers. The length and width of 2975 cells were measured along a 38-year transect (1985&ndash;2022) using stitched scanning electron microscope images. Cell length declined significantly over time (<em>R</em>²_adj = 0.105,<em> p</em> = 0.027), whereas cell width showed no temporal trend (<em>R</em>²_adj = -0.019, <em>p</em> = 0.583). Among the environmental variables, SSR was the strongest predictor of cell length (<em>R</em>²_adj = 0.235, <em>p</em> = 0.001), while SST and <em>p</em>CO₂ explained comparatively little variance. These results identify light availability as a primary correlate of cell elongation in <em>S. nodosum</em>, consistent with a role for irradiance-driven changes in photosynthetic energy supply. Because long-term SSR trends are spatially heterogeneous, light-mediated shifts in CCA cell dimensions &ndash; and their potential implications for rhodolith structure and ecosystem function &ndash; are likely to vary regionally. This study highlights the central role of light availability in shaping CCA cell morphology under changing surface-ocean conditions and motivates multi-site comparisons to assess broader ecosystem implications.