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Ternary computers use three-valued logic in their calculations. History has several examples of this form of computing.

One of the earliest calculating machines, built by Thomas Fowler entirely from wood in 1840, was a ternary computer. The largest-yet ternary computer (called Setun) was built in the late 1950s in the Soviet Union at the Moscow State University, and it had notable advantages to the binary computers which eventually replaced it.

With the advent of mass-produced binary components for computers, ternary computers have diminished to a small footnote in the history of computing. However, ternary logic's elegance and efficiency is predicted by Donald Knuth^{[citation needed]} to bring them back into development in the future.

Ternary computing is commonly implemented in terms of balanced ternary, which uses the three digits -1, 0, and +1. The negative value of any balanced ternary number can be obtained by replacing every + with a – and vice versa. It is easy to subtract a number by inverting the + and – digits and then using normal addition. Balanced ternary can express negative values as easily as positive ones, without the need for a leading negative sign as with decimal numbers. These advantages make some calculations more efficient in ternary than binary.

"I often reflect that had the Ternary instead of the binary Notation been adopted in the Infancy of Society, machines something like the present would long ere this have been common, as the transition from mental to mechanical calculation would have been so very obvious and simple." (Fowler, 1840)

• In Robert A. Heinlein's novel Time Enough for Love, the sentient computers, including Minerva, use a ternary system, although it is not specified whether their math is balanced.

• Ternary logic
• Ternary numeral system
• Balanced ternary
• Setun

• Development of ternary computers at Moscow State University
• The ternary calculating machine of Thomas Fowler
• Trinary.cc - logic gates used to build a ternary computer