The tsunami source of the 2021 MW 8.1 Raoul Island earthquake in the Kermadec subduction zone was estimated by inverting the tsunami signals recorded by Deep‐ocean Assessment and Reporting of Tsunamis (DART) bottom pressure sensors and coastal tide‐gauges. The main asperity of up to 5 m of slip is located northeastward from the hypocenter, with features compatible with the aftershock distribution and rapid back‐projection analysis. Three earthquakes of MW ∼8 or larger which also produced moderate tsunamis happened in the 20th century in the same portion of the subduction zone. This is the first great tsunamigenic event captured by the new New Zealand DART network in the South West Pacific, which proved valuable to estimate a robust image of the tsunami source. We also show a first proof of concept of the capability of this network to reduce the uncertainty associated with tsunami forecasting and to increase the lead time available for evacuation for future alerts. Plain Language Summary We estimated the tsunami source for the 4 March 2021 Raoul Island earthquake (MW 8.1), obtained by inverting tsunami data from tide‐gauges and open ocean Deep‐ocean Assessment and Reporting of Tsunamis (DART) stations. The main asperity of up to 5 m of slip is located northeastward from the hypocenter, with features compatible with the aftershock distribution and rapid back‐projection analysis. This event is important because it was the strongest one of three earthquakes that occurred within hours during the same day. Moreover, it caused the largest of three tsunami that altogether represent a great test for the New Zealand DART new network. The results demonstrate the potential importance of this new DART network for resolving the tsunami source and for early warning purposes as it can reduce the uncertainty of the tsunami forecasts and at the same time increase the lead time available for evacuation. Key Points Tsunami source of the 2021 MW 8.1 Raoul Island earthquake by inverting tsunami waveforms The main slip peaks at 5 m and is located at a depth of ∼20–30 and ∼100 km north of the epicenter New Deep‐ocean Assessment and Reporting of Tsunamis network was crucial for characterizing the source and will significantly reduce the uncertainty and speed up future warnings