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Ennis, who has been the face of the Games, finished like a champion by winning the 800 metres for a combined total of 6,955 points for the seven disciplines that make up the event.
Russian Tatyana Chernova, who beat Ennis to the world title last year, moved up from sixth to take the silver and Ukraine's Lyudmyla Yosypenko finished with the bronze.
Ennis, who missed the Beijing Games through injury, led the field following Friday's (local time) four events and all but secured the gold after finishing second to Chernova in the long jump and setting a personal best of 47.49 in her weakest event, the javelin.
Going into the 800, Lithuania's Austra Skujyte was in second overall but she struggled around the two laps and crossed the line in last place, almost 12 seconds behind Ennis.
Ennis, 26, threw her arms aloft as she finished in a rare show of emotion after keeping her composure throughout the two-day competition.
Clearly struggling with her emotions, she draped herself in a Union Jack flag and led the traditional joint lap of honour by all the heptathletes around a cheering stadium.