Development of multifunctional theranostics is of great significance for cancer management. Herein, we develop polyethylene glycol (PEG) modified cobalt carbide nanoparticles (Co2C-PEG NPs) as cancer photothermal theranostic agents for multimodal imaging and photothermal therapy (PTT). Co2C NPs are synthesized by a high-temperature thermal decomposition method. Afterwards, the morphology, photothermal effect, multimodal imaging capacities, biocompatibility, and PTT efficacy of Co2C-PEG NPs are carefully investigated. The as-prepared Co2C-PEG NPs exhibit high photothermal conversion efficiency (PCE, η = 35.7%) and good photostability. Through photoacoustic (PA), magnetic resonance (MR), and photothermal (PT) tri-modal imaging, in vivo pharmacokinetics and tumor temperature elevation could be monitored during the PTT process. Meanwhile, the Co2C-PEG NPs also show good PTT efficacy both in vitro and in vivo. Our findings suggest that Co2C-PEG NPs are effective for cancer photothermal theranostics.