Reflectance changes at 531 nm, associated with the zeaxanthin‐antheraxanthin‐violaxanthin interconversion and the related thylakoid energization, are widespread among plant species. We evaluated an index based on 531 nm reflectance (‘PRI’, Photochemical Reflectance Index calculated as (R₅₃₁− R₅₇₀)/(R₅₃₁+ R₅₇₀)) as an indicator of efficiency of photosynthetic radiation use in seven species representing both C₃ and CAM photosynthetic pathways. Leaves exposed to a dark‐lighted ark transition in a steady‐state laboratory gas exchange system exhibited nearly parallel changes in PRI and PS II quantum yield (ΔF/F_m′). Similar PRI and ΔF/F_m′ responses were seen in leaves exposed to diurnally changing sunlight levels outdoors. PRI was linearly related to ΔF/F_m′, and both ΔF/F_m′ and PRI were exponentially related to instantaneous efficiency of photosynthetic radiation‐use in different species over a range of different field conditions. These results extend previous studies by indicating a functional relationship between PRI. ΔF/F_m′, and photosynthetic radiation‐use efficiency. The narrow‐band PRI index offers a simple, portable means of assessing PS II radiation‐use efficiency, analogous to ΔF/F_m′, and with the potential for remote applications at scales larger than the leaf.