# Callosum raises $100M seed round led by Atomico

> Callosum, based in United Kingdom, has raised $100M in a seed round led by Atomico, announced on 20 August 2026. 4 other investors joined.

Source: https://millionrounds.com/news/rounds/callosum-seed-round-2026-08 (Millionrounds; every figure on the page links to its filing or article)

Raised **$100M** 6 sources reported it

Stage **Seed** 2 earlier rounds on record

Investors **5** 1 named as lead

Where **GB** United Kingdom

AI Construction

## 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.

**#418** equity round of the last twelve months

**#15** seed round of 695 in that time

**#30** round 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

- [**Atomico** 34 rounds in the last twelve months *Led*](https://millionrounds.com/fund/atomico)

Also in the round

- [DCVC](https://millionrounds.com/fund/data-collective)
- [Plural](https://millionrounds.com/fund/plural)
- [AlbionVC](https://millionrounds.com/fund/albionvc)
- [UK Sovereign AI Fund](https://millionrounds.com/fund/uk-sovereign-ai-fund)

## Funding history

- August 2026 Seed **$100M**
- April 2026 Venture **Undisclosed**
- February 2026 Pre-seed **$10M**

$110M raised across the 3 rounds we hold.

## Reported by

- AlbionVC 20 August
- TechStartups 20 August
- Ventureburn 20 August
- shattered.io 26 October
- tech-insider.org 26 October
- Tech.eu 26 October

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

## Other big rounds that week

[**Starcloud** $250M Series A](https://millionrounds.com/news/rounds/starcloud-series-a-2026-08)[**Castelion** $800M Series C](https://millionrounds.com/news/rounds/castelion-series-c-2026-08)[**Muon Space** $250M Series C](https://millionrounds.com/news/rounds/muon-space-series-c-2026-08)[**Groq** $350M Series A](https://millionrounds.com/news/rounds/groq-series-a-2026-08)[**Also** $150M Series D](https://millionrounds.com/news/rounds/also-series-d-2026-08)[W **Wispr** $280M Series B](https://millionrounds.com/news/rounds/wispr-series-b-2026-08)

[The company **Callosum on Millionrounds** *→*](https://millionrounds.com/company/callosum)[Weekly roundup **Week of 17 August** *→*](https://millionrounds.com/news/weekly/2026-08-17)[Monthly roundup **August 2026** *→*](https://millionrounds.com/news/monthly/2026-08)

**How this article is made**

We write one for every equity round of $100M or more that at least two sources reported. It is put together from our record of rounds, not rewritten from the reports: the ranks and medians count every equity round we read in the twelve months to the round's date. [How we read rounds](https://millionrounds.com/methodology) · [Report a mistake](https://millionrounds.com/corrections)