Big round · 20 August 2026

Callosum raises $100M seed round led by Atomico

By Millionrounds ResearchPublished 16 September 2026Updated 26 October 2026

Callosum, based in United Kingdom, has raised $100M in a seed round led by Atomico, announced on 20 August 2026. 4 other investors joined. It is the 418th largest equity round we recorded in the twelve months to that date, and the 15th largest seed round of 695. The median seed round over the same period was $5.8M, so this one is 17 times that. Its previous round on record was a venture round in April 2026, 5 months earlier.

Raised$100M6 sources reported it
StageSeed2 earlier rounds on record
Investors51 named as lead
Where GBUnited Kingdom

AIConstruction

How big is it?

Against every equity round we recorded in the twelve months to 20 August 2026. Ranks count companies, so the same raise reported twice is counted once.

#418equity round of the last twelve months
#15seed round of 695 in that time
#30round in United Kingdom, of 296
This round$100M
Median seed round$5.8M

17 times the median seed round of the last twelve months.

Who invested

Also in the round

Funding history

  1. August 2026Seed$100M
  2. April 2026VentureUndisclosed
  3. February 2026Pre-seed$10M

$110M raised across the 3 rounds we hold.

Reported by

  1. AlbionVC20 August
  2. TechStartups20 August
  3. Ventureburn20 August
  4. shattered.io26 October
  5. tech-insider.org26 October
  6. Tech.eu26 October

The round's figures come from these reports; each link opens the original.

Other big rounds that week

Starcloud$250M Series ACastelion$800M Series CMuon Space$250M Series CGroq$350M Series AAlso$150M Series DWWispr$280M Series B
The companyCallosum on Millionrounds→Weekly roundupWeek of 17 August→Monthly roundupAugust 2026→