A novel approach to transient stability analysis in multi-terminal high voltage direct current (MTDC) grids is presented in this paper. A symmetrical three-phase fault in an ac grid connected to a rectifier terminal of the MTDC grid causes the power injected into the dc grid to decrease, which in turn leads to a lower dc voltage in the MTDC grid. If dc voltage drops below a critical voltage limit before the ac fault is cleared, then the dc grid becomes unstable and its operation is disrupted. An analytical approach is proposed in this paper to calculate the critical clearing time of a fault in an ac grid behind a rectifier terminal beyond which dc voltage collapse occurs. A five-terminal MTDC grid modeled in EMTDC/PSCAD is used to validate the results obtained with the analytical method.