Thirteen readily accessible azo D–π-A dyes, intended for use as photothermal agents, were synthesized using only a few steps. Absorption wavelengths were readily tuned by changing the building blocks, and 6 of these dyes exhibited NIR absorption that would be useful for biomedical applications. Unexpected suppression of an N–C single bond rotation that neighbors the azo bond was observed in the case of 5 dyes. Photothermal conversion efficiency measurements revealed a significant effect of the D moiety in these synthesized azo D–π-A dyes, but neither the π moiety nor the A moiety showed an obvious influence. The obtained results offer valuable information for the design of high-performance azo D–π-A dyes that have utility as photothermal agents.