The role of North Pacific bomb cyclones in the onset of January 2021 sudden stratospheric warming (SSW) is examined by conducting a set of numerical model experiments. The control simulation, initialized 10 days before the SSW onset, successfully reproduces the SSW. As this event is preceded by the bomb cyclones in the North Pacific, their impact is tested by initializing the model without them. This sensitivity experiment shows much weaker polar‐vortex deceleration than the control simulation, resulting in no distinct SSW onset. This difference is attributable to the dampened constructive linear interference between the climatological wave and the cyclone‐related wavenumber‐one anomaly in the sensitivity experiment. It weakens the vertical propagation of wavenumber‐one wave into the stratosphere, thereby reducing wave breaking in the polar stratosphere. This result suggests that bomb cyclones should be considered for better understanding SSW and improving its predictability. Plain Language Summary Sudden stratospheric warming (SSW) is a dramatic event characterized by abrupt warming in the polar stratosphere. During this event, the eastward winds encircling the polar stratosphere change to westward winds, due to the breaking of the upward‐propagating planetary‐scale waves. Since the SSW is often followed by unusual weather events (e.g., cold surges), an accurate prediction of SSW is important for a better weather prediction. Here, we examine the role of the rapidly developing North Pacific cyclones in initiating the 2021 SSW. A set of numerical model experiments show that without cyclones, the 2021 SSW is not initiated due to the reduced breaking of the upward‐propagating planetary‐scale waves. This result suggests that the rapidly developing North Pacific cyclones should be considered for better understanding SSW and improving its predictability. Key Points The 2021 SSW is successfully reproduced by a model initialized 10 days before the stratospheric warming (SSW) The model initialized without the North Pacific bomb cyclone does not simulate the 2021 SSW Removing the North Pacific bomb cyclone reduces the k = 1 wave propagation into the stratosphere, reducing wave breaking in the stratosphere