In this Letter, an imaging spectrometer in which a freeform concave grating is the only optical component in the system is introduced. The degrees of freedom of optical freeform surfaces and a variable line-spacing (VLS) grating are used to realize imaging spectrometers. A point-by-point system design method is proposed that can generate a good initial solution rapidly. By exploring the limitations of the system specifications, it is demonstrated that the spectral dispersion, spectral resolving power, and system length can be improved significantly by using the freeform VLS concave grating. It is also found that freeform surfaces with higher degrees of freedom than a toroid can further improve system performance when using a VLS grating.