Grignard reagents or Grignard compounds are  chemical compounds with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl.  Two typical examples are methylmagnesium chloride  and phenylmagnesium bromide . They are a subclass of the organomagnesium compounds.
Grignard compounds are popular reagents in organic synthesis for creating new carbon–carbon bonds. For example, when reacted with another halogenated compound  in the presence of a suitable catalyst, they typically yield  and the magnesium halide  as a byproduct; and the latter is insoluble in the solvents normally used.  In this aspect, they are similar to organolithium reagents.
Grignard reagents are rarely isolated as solids.  Instead, they are normally handled as solutions in solvents such as diethyl ether or tetrahydrofuran using air-free techniques. Grignard reagents are complex with the magnesium atom bonded to two ether ligands as well as the halide and organyl ligands. 
The discovery of the Grignard reaction in 1900 was recognized with the Nobel Prize awarded to Victor Grignard in 1912.