Which fictional corporation developed the T-virus in the Resident Evil series?

Umbrella Corporation developed the T-virus in the Resident Evil series.

Umbrella is a multinational pharmaceutical company that secretly researches biological weapons. Its scientists develop the T-virus from the Progenitor virus, and the resulting pathogen becomes the central cause of the outbreak connected to Spencer Mansion and Raccoon City.

The corporation's public image as a pharmaceutical business contrasts with its hidden weapons program. This double identity is one of the foundational ideas of the Resident Evil setting. Umbrella's research produces zombies, mutant animals, and larger bio-organic weapons such as the Tyrant.

Other companies also become important later. Tricell supplies or studies bioweapons in Resident Evil 5, WilPharma appears in the wider series timeline, and Neo-Umbrella is a separate organization in Resident Evil 6. None of those is the original developer of the T-virus.