In geometry, a hendecagon (also undecagon[1][2] or endecagon[3]) or 11-gon is an eleven-sided polygon.

[6] Because 11 is not a Pierpont prime, construction of a regular hendecagon is still impossible even with the usage of an angle trisector.

[9] The following construction description is given by T. Drummond from 1800:[10] Draw the radius A B, bisect it in C—with an opening of the compasses equal to half the radius, upon A and C as centres describe the arcs C D I and A D—with the distance I D upon I describe the arc D O and draw the line C O, which will be the extent of one side of a hendecagon sufficiently exact for practice.On a unit circle: The regular hendecagon has Dih11 symmetry, order 22.

Cyclic symmetries in the middle column are labeled as g for their central gyration orders.

The United States Susan B. Anthony dollar has a hendecagonal outline along the inside of its edges.