Phosphorus pentabromide is the inorganic compound with the formula PBr₅. It is a yellow ionic solid that is used in organobromine compounds. The compound participates in diverse equilibria, especially in the presence of additional bromine.

Unlike phosphorus pentachloride, solid phosphorus pentabromide is an ionic compound. Its structural formula is [PBr₄]⁺Br⁻ (tetrabromophosphonium bromide). The [PBr₄]⁺] cation is tetrahedral. According to X-ray crystallography, the P-Br bond lengths are 215 picometers.

In the vapor phase, it dissociates into PBr₃ and Br₂. Rapid cooling of the vapor to 15 K gives a mixture of phosphorus tribromide and the similarly ionic phosphorus heptabromide (tetrabromophosphonium tribromide or [PBr₄]⁺[Br₃]⁻).

PBr₅ can be synthesized with high purity by bromination of a solution of PBr₃ in CS₂:

Addition of Br₂ to PBr₃ is complicated because phosphorus pentabromide can further convert to phosphorus heptabromide, which features the tribromide ion:

Phosphorus pentabromide is used in organic chemistry to convert carboxylic acids to acyl bromides.

Phosphorus pentabromide is a skin irritant and a source of bromine, a strong oxidant.

Triphenylphosphine dibromide, (C₆H₅)₃PBr₂