Encyclopedias, Almanacs, Transcripts, and Maps
- Thomas, Lee M. (1944 – ) American Former Environmental Protection Agency Administrator
- Thomas, R.S.
- Thompson, Benjamin (Count Rumford) (1753–1814)
- Thomson, Joseph John (1856–1940)
- Thoreau, Henry David (1817 – 1862) American Writer and Natural Philosopher
- thorn apple
- Thracian Religion
- Thracian Rider
- Thrashers and Mockingbirds (Mimidae)
- Three Gorges Dam
- Three Mile Island Nuclear Reactor
- Three-Toed Toadlets (Brachycephalidae)
- Three-Toed Tree Sloths (Bradypodidae)
- Threshold Dose
- thrip
- Thrushes and Chats (Turdidae)
- Thunderstorms
- Thysanoptera (Thrips)
- Thysanura (Silverfish and fire brats)
- Tian