The women's 78 kg category in judo at the 2012 Olympic Games in London took place between 28 July and 2 August at the ExCeL Exhibition Centre.

[1] The gold and silver medals were determined by a single-elimination tournament, with the winner of the final taking gold and the loser receiving silver.

Judo events awarded two bronze medals.

Quarter-final losers competed in a repechage match for the right to face a semi-final loser for a bronze medal (that is, the judokas defeated in quarter-finals 'A' and 'B' competed against each other, with the winner of that match facing the semi-final loser from the other half of the bracket).

[1] Kayla Harrison, representing the United States, won the gold medal, defeating Great Britain's Gemma Gibbons in the final; Gibbons was awarded the silver medal.