This paper describes identification of the first-in-class multi-target adenosine A receptor antagonists-carbonic anhydrase (CA) IX and XII inhibitors, as new potential antitumor agents. To obtain the multi-acting ligands, the 8-amino-2,6-diphenyltriazolo[4,3-a]pyrazin-3-one, a potent adenosine hA receptor (AR) antagonist, was taken as lead compound. To address activity against the tumor-associated CA isoforms, it was modified by introduction of different substituents (OH, COOH, CONHOH, SONH) on the 6-phenyl ring or on a phenyl pendant connected to the former through different spacers. Among the new triazolopyrazines 1-23, the most active were those featuring the sulfonamide residue. Derivative 20, featuring a 4-sulfonamidophenyl residue attached through a CONH(CH)CONH spacer at the para-position of the 6-phenyl ring, showed the best combination of activity at the three targets. In fact, it inhibited both the tumor-associated hCA IX and XII isozymes at nanomolar concentration (K = 5.0 and 27.0 nM), and also displayed a quite good affinity for the hA AR (K = 108 nM). Compound 14, bearing the 4-sulfonamidophenyl residue linked at the para-position of the 6-phenyl ring by a CONH spacer, was remarkable because both its hA AR affinity and hCA XII inhibitory potency were in the low nanomolar range (K = 6.4 and 6.2 nM, respectively). Molecular docking studies highlighted the interaction mode of selected triazolopyrazines in the hA AR recognition pocket and in the active site of hCA II, IX and XII isoforms.