We present a colloidal route for the synthesis of ultrathin ZrS2 (UT-ZrS2) nanodiscs that are 1.6 nm thick and consist of approximately two unit cells of S–Zr–S. The lateral size of the discs can be tuned to 20, 35, or 60 nm while their thickness is kept constant. Under the appropriate conditions, these individual discs can self-assemble into face-to-face-stacked structures containing multiple discs. Because the S–Zr–S layers within individual discs are held together by weak van der Waals interactions, each UT-ZrS2 disc provides spaces that can serve as host sites for intercalation. When we tested UT-ZrS2 discs as anodic materials for Li+ intercalation, they showed excellent nanoscale size effects, enhancing the discharge capacity by 230% and greatly improving the stability in comparison with bulk ZrS2. The nanoscale size effect was especially prominent for their performance in fast charging/discharging cycles, where an 88% average recovery of reversible capacity was observed for UT-ZrS2 discs with a lateral diameter of 20 nm. The nanoscale thickness and lateral size of UT-ZrS2 discs are critical for fast and reliable intercalation cycling because those dimensions both increase the surface area and provide open edges that enhance the diffusion kinetics for guest molecules.