The Peterloo Massacre (or Battle of Peterloo) occurs at St. Peter's Field, Manchester, England, on August 16, 1819, when cavalry charges into a crowd of sixty thousand to eighty thousand that has gathered to demand the reform of parliamentary representation.

The end of the Napoleonic Wars in 1815 had resulted in periods of famine and chronic unemployment, exacerbated by the introduction of the first of the Corn Laws.

By the beginning of 1819 the pressure generated by poor economic conditions, coupled with the lack of suffrage in northern England, has enhanced the appeal of political radicalism.

In response, the Manchester Patriotic Union, a group agitating for parliamentary reform, organizes a demonstration to be addressed by the well-known radical orator Henry Hunt.Shortly after the meeting began, local magistrates call on the military authorities to arrest Hunt and several others on the hustings with him, and to disperse the crowd.

Cavalry charges into the crowd with sabers drawn, and in the ensuing confusion, fifteen people are killed and four hundred to seven hundred are injured.

The massacre is given the name Peterloo in ironic comparison to the Battle of Waterloo, which had taken place four years earlier.In its own time, the London and national papers shares the horror felt in the Manchester region, but Peterloo's immediate effect is to cause the government to crack down on reform, with the passing of what became known as the Six Acts.

It also leads directly to the foundation of The Manchester Guardian (now The Guardian), but has little other effect on the pace of reform.

In a survey conducted by The Guardian in 2006, Peterloo came second to the Putney Debates as the event from British history that most deserved a proper monument or a memorial.

Peterloo is commemorated by a plaque close to the site, a replacement for an earlier one that was criticized as being inadequate as it did not reflect the scale of the massacre.