In geometry, a **120-gon** is a polygon with 120 sides. The sum of any 120-gon's interior angles is 21240 degrees.

Alternative names include **dodecacontagon** and **hecatonicosagon**.

A *regular 120-gon* is represented by Schläfli symbol {120} and also can be constructed as a truncated hexacontagon, t{60}, or a twice-truncated triacontagon, tt{30}, or a thrice-truncated pentadecagon, ttt{15}.

One interior angle in a regular 120-gon is 177°, meaning that one exterior angle would be 3°.

The area of a regular 120-gon is (with *t* = edge length)

A
=
30
t
2
cot
π
120
and its inradius is

r
=
1
2
t
cot
π
120

The circumradius of a regular 120-gon is

R
=
1
2
t
csc
π
120
This means that the trigonometric functions of π/120 can be expressed in radicals.

Since 120 = 2^{3} × 3 × 5, a regular 120-gon is constructible using a compass and straightedge. As a truncated hexacontagon, it can be constructed by an edge-bisection of a regular hexacontagon.

The *regular 120-gon* has Dih_{120} dihedral symmetry, order 240, represented by 120 lines of reflection. Dih_{120} has 15 dihedral subgroups: (Dih_{60}, Dih_{30}, Dih_{15}), (Dih_{40}, Dih_{20}, Dih_{10}, Dih_{5}), (Dih_{24}, Dih_{12}, Dih_{6}, Dih_{3}), and (Dih_{8}, Dih_{4}, Dih_{2}, Dih_{1}). And 16 more cyclic symmetries: (Z_{120}, Z_{60}, Z_{30}, Z_{15}), (Z_{40}, Z_{20}, Z_{10}, Z_{5}), (Z_{24}, Z_{12}, Z_{6}, Z_{3}), and (Z_{8}, Z_{4}, Z_{2},Z_{1}), with Z_{n} representing π/*n* radian rotational symmetry.

These 32 symmmetries are related to 44 distinct symmetries on the 120-gon. John Conway labels these lower symmetries with a letter and order of the symmetry follows the letter. He gives **d** (diagonal) with mirror lines through vertices, **p** with mirror lines through edges (perpendicular), **i** with mirror lines through both vertices and edges, and **g** for rotational symmetry. **a1** labels no symmetry.

These lower symmetries allows degrees of freedom in defining irregular 120-gons. Only the **g120** symmetry has no degrees of freedom but can seen as directed edges.

A 120-gram is a 120-sided star polygon. There are 15 regular forms given by Schläfli symbols {120/7}, {120/11}, {120/13}, {120/17}, {120/19}, {120/23}, {120/29}, {120/31}, {120/37}, {120/41}, {120/43}, {120/47}, {120/49}, {120/53}, and {120/59}, as well as 44 compound star figures with the same vertex configuration.