Carbon dots (CDs) have garnered significant interest owing to their distinctive optical properties. However, their bioimaging and biomedical applications are limited by pronounced fluorescence (FL) quenching in aqueous media and low tumor accumulation efficacy associated with their ultra-small size. This study proposes a simple surface modification approach using functioning D-arginine on CDs (D-Arg@CDs) to improve their near-infrared (NIR) FL in aqueous solution and maintain their high photothermal conversion properties. Because of the low utilization rate of dextral amino acids in animals, modifying CDs with low molecular weight D-arginine did not increase particle size but extended the metabolism time in blood circulation, thereby leading to enhanced accumulation efficacy at tumor sites in the mice model. The enhanced tumor accumulation of D-Arg@CDs resulted in significantly superior tumor NIR FL imaging and photothermal therapy performance compared with pure CDs and L-arginine functionalized CDs. This dextral amino acid modification approach is expected to be an effective tool for enhancing the biomedical applications of CDs.