The fifteenth named storm of the 2021 Pacific typhoon season, Dianmu originated from an area of low-pressure situated in the South China Sea on September 21.

On September 21 at 21:00 UTC, the United States Joint Typhoon Warning Center (JTWC)[nb 1] started to monitor an area of low-pressure in the South China Sea, 168 nmi (311 km; 193 mi) to the northwest of Puerto Princesa, Philippines.

Environmental analysis on the area depicted a favorable path for tropical cyclogenesis, with low wind shear, good outflow and warm 29–30 °C (84–86 °F) sea surface temperatures.

[10][11] The JMA, meanwhile, downgraded Dianmu back to a tropical depression twelve hours later before it issued its final warning on the system as it dissipated as a remnant low at 12:00 UTC on September 24.

There, preparations discussed included the sending information to the remaining 28 ships in the waters off Quang Binh to Bình Thuận and the Covid-19 testing of vessel crews and sheltering their boats.

Evacuation of individuals in Quảng Nam were discussed, along with the advice of the People's Committee of the province to store food and important necessities for 7–10 days.

[21] The system also likely spawned two tornados in the country, one in Núi Thành District, Quảng Nam Province on September 23 in which it caused roof and household damages, including a school.

[19][23] A 2,000-ton cargo ship collided with a fishing boat on the coast of Bình Định Province on September 24, leaving 2 fishermen missing out of the three of the latter which was rescued.

[26] As of September 25, over 2,845 families (14,225 individuals) were affected by Dianmu, 2725 houses and 3,734 ha of agricultural crops and lands being damaged, 201 roads and 5 schools suffering significant destruction and 5,402 livestock being reported to have been killed.

[27] In Laos, the National Disaster Management Organisastion (NDMO) reported that Dianmu affected several provinces in the country, although they are still continuing to assess the damages from the storm there as of September 28.