十八边形是几何学中所有有18条边及18只角的多边形。

正十八边形是有18边的正多边形。正十八边形的每个内角为160°。

正十八边形不能仅用尺规作出。以下给出一个误差为0.0005°的近似作图：

An octadecagon (or octakaidecagon) is an eighteen-sided polygon or 18-gon.

A regular octadecagon has a Schläfli symbol {18} and can be constructed as a quasiregular truncated enneagon, t{9}, which alternates two types of edges.

As 18 = 2 × 3, a regular octadecagon cannot be constructed using a compass and straightedge. However, it is constructible using neusis, or an angle trisector. The following approximate construction is very similar to that of the enneagon, as an octadecagon can be constructed as a truncated enneagon. It is also feasible with exclusive use of compass and straightedge.