Among human carbonic anhydrase (CA) inhibitors, the α,γ-diketocarboxylic acids and esters are still poorly investigated. Here, we report the first compounds of this class (1-6) acting as potent inhibitors at low nanomolar level against the cancer-related human CA IX and XII, and 2-3 magnitude orders selective toward the cytosolic isoforms hCA I and II. At enzymatic level, the α,γ-diketoacids 1-3 were more effective inhibitors compared to the corresponding ethyl esters 4-6. The phenyl- and α-naphthyl-containing compounds (1, 3, 4, and 6) behaved as dual hCA IX/XII inhibitors, while the β-naphthyl analogues (2 and 5) exhibited hCA IX-selective inhibition. In MG63 and HOS osteosarcoma (OS) cell lines, the ethyl esters 5 and 6 displayed dose-dependent reduction of viability and proliferation after 72 h treatment, with 6 being more potent than 5 likely for its dual hCA IX/XII inhibition.