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Organic molecules with second near‐infrared (NIR‐II, 1000–1700 nm) emission have significant advantages over NIR‐I (600–900 nm) dyes, and have been widely used as phototheranostic agents (PTAs) and fluorescent probes. However, the two biggest challenges for such reagents are improving photothermal conversion efficiency (PCE), and simplifying tedious synthetic procedures. Herein, different from the traditional NIR skeleton, for the first time, it develops two novel dibenzofulvene‐based NIR‐II emission PTAs via D‐π‐A concept with high PCEs through two simple synthetic steps. The increased intramolecular charge transfer effect and extended π‐conjugation effectively red‐shift their emissions to the NIR‐II region. In addition, the introduction of molecular rotors/vibrators and the formation of π–π stacking prompt the two PTAs to exhibit high PCEs. Notably, FE‐IDMN nanoparticles (FE‐IDMN NPs) achieve an exciting PCE of 82.6%, higher than most reported photothermal agents with NIR‐II emission. FE‐IDMN NPs also possess good colloidal, pH, and photothermal stabilities, as well as excellent photoacoustic response, which is further successfully applied for NIR‐II fluorescence/photoacoustic/photothermal imaging‐guided photothermal therapy. This work provides a simple strategy for constructing new D‐π‐A PTAs with low molecular weight, NIR‐II emission, and high PCE for cancer therapy. [ABSTRACT FROM AUTHOR]