Bicyclo[1.1.1]pentane is an organic compound, the simplest member of the bicyclic bridged compounds family. It is a hydrocarbon with formula C<sub>5</sub>H<sub>8</sub>. The molecular structure consists of three rings of four carbon atoms each.
Bicyclo[1.1.1]pentane is a highly strained molecule.
Kenneth Wiberg and Frederick Walker used a bicyclo[1.1.1]pentane derivative to make [1.1.1]propellane. Conversely, some substituted bicyclo[1.1.1]pentanes are best synthesized from the corresponding propellanes.
The bicyclo[1.1.1]pentane structure has been used as an unusual bioisostere for a phenyl ring.