In geometry, a triakis octahedron (or trigonal trisoctahedron or kisoctahedron) is an Archimedean dual solid, or a Catalan solid. Its dual is the truncated cube.

It can be seen as an octahedron with triangular pyramids added to each face; that is, it is the Kleetope of the octahedron. It is also sometimes called a trisoctahedron, or, more fully, trigonal trisoctahedron. Both names reflect that it has three triangular faces for every face of an octahedron. The tetragonal trisoctahedron is another name for the deltoidal icositetrahedron, a different polyhedron with three quadrilateral faces for every face of an octahedron.

This convex polyhedron is topologically similar to the concave stellated octahedron. They have the same face connectivity, but the vertices are at different relative distances from the center.

If its shorter edges have length of 1, its surface area and volume are:

Let α = √2 − 1, then the 14 points (±α, ±α, ±α) and (±1, 0, 0), (0, ±1, 0) and (0, 0, ±1) are the vertices of a triakis octahedron centered at the origin.

The length of the long edges equals √2, and that of the short edges 2√2 − 2.

The faces are isosceles triangles with one obtuse and two acute angles. The obtuse angle equals arccos(⁠1/4⁠ − ⁠√2/2⁠) ≈ 117.20057038016° and the acute ones equal arccos(⁠1/2⁠ + ⁠√2/4⁠) ≈ 31.39971480992°.

The triakis octahedron has three symmetry positions, two located on vertices, and one mid-edge: