A photoacoustic contrast agent that is based on bis-carboxylic acid derivative of indocyanine green (ICG) covalently conjugated to single-wall carbon nanotubes (ICG/SWCNT) is presented. Covalently attaching ICG to the functionalized SWCNT provides a more robust system that delivers much more ICG to the tumor site. The detection sensitivity of the new contrast agent in a mouse tumor model is demonstrated in vivo by our custom-built photoacoustic imaging system. The summation of the photoacoustic tomography (PAT) beam envelope, referred to as the "PAT summation," is used to demonstrate the postinjection light absorption of tumor areas in ICG- and ICG/SWCNT-injected mice. It is shown that ICG is able to provide 33% enhancement at approximately 20 min peak response time with reference to the preinjection PAT level, while ICG/SWCNT provides 128% enhancement at 80 min and even higher enhancement of 196% at the end point of experiments (120 min on average). Additionally, the ICG/SWCNT enhancement was mainly observed at the tumor periphery, which was confirmed by fluorescence images of the tumor samples. This feature is highly valuable in guiding surgeons to assess tumor boundaries and dimensions in vivo and to achieve clean tumor margins to improve surgical resection of tumors.