Aza-BODIPY dyes have recently come to attention owing to their excellent chemical and photophysical properties. In particular, their absorption and emission maxima can efficiently be shifted to the red or even to the NIR spectral region. On this basis, aza-BODIPY derivatives are widely investigated as fluorescent probes or phototherapeutic agents. Here we report the synthesis of a set of novel aza-BODIPY derivatives as potential photosensitizers for use in photodynamic therapy. Triazolyl derivatives were obtained via Cu(I)-catalyzed azide-alkyne cycloaddition as the key step. In vitro photodynamic activities of the newly synthesized compounds were evaluated on the A431 human epidermoid carcinoma cell line. Structural differences influenced the light-induced toxicity of the test compounds markedly. Compared to the initial tetraphenyl aza-BODIPY derivative, the compound bearing two hydrophilic triethylene glycol side chains showed substantial, more than 250-fold, photodynamic activity with no dark toxicity. Our newly synthesized aza-BODIPY derivative, acting in the nanomolar range, might serve as a promising candidate for the design of more active and selective photosensitizers.