Innovations in opportunistic frame dropping for variable-frame-rate encoding of digital video are presented. In general, a computing system selectively drops a frame when the cost of encoding the frame (e.g., in terms of use of computational resources and/or power) is expected to outweigh the benefit of encoding the frame (e.g., in terms of better quality). For example, a frame dropping module detects whether there is significant change in a given frame relative to a control frame, which is a previous frame stored in a control frame buffer. If significant change is detected, the frame dropping module stores the given frame in the control frame buffer, thereby replacing the control frame, and passes the given frame to a video encoder. Otherwise, the frame dropping module drops the given frame without replacing the control frame in the control frame buffer and without passing the given frame to the video encoder.