Two-dimensional (2D) polymers are fascinating as they exhibit unique physical, chemical, mechanical, and electronic properties that are completely different from those of traditional linear or branched polymers. They are very promising for applications in catalysis, separation, optoelectronics, energy storage, and nanomedicine. Recently, ultrathin 2D conjugated polymers have emerged as advanced materials for converting solar energy into chemical energy. The inherent 2D planar structure with in-plane periodicity offers many features that are highly desirable for photon-involved catalytic energy conversion processes, including high absorption coefficients, large surface areas, abundant surface active sites, and efficient charge separation. Moreover, the possibility of finely tuning the optoelectronic and structural properties through precise molecular engineering has opened up new opportunities for design and synthesis of novel 2D polymer nanosheets with unprecedented applications. Herein, we highlight recent advances in developing ultrathin 2D conjugated polymer nanosheets for solar-to-chemical energy conversion. Specifically, we discuss emerging applications of ultrathin 2D conjugated polymer nanosheets for solar-driven water splitting and CO2 reduction. Meanwhile, future challenges and prospects for design and synthesis of ultrathin 2D conjugated polymer nanosheets for solar fuel generation are also included.