Marietta Edgecombe is a fictional character in Harry Potter and the Order of the Phoenix. Marietta Edgecomb has curly, reddish-blonde hair and is a friend of Cho Chang. In Harry Potter and the Goblet of Fire, it was stated that Cho was always followed by a group of giggling girls, but Marietta is the only one who has been given a name or described in any detail.

Marietta's mother is Madam Edgecombe, who helped Cornelius Fudge and Dolores Umbridge take control of Hogwarts. Madam Edgecombe works in the Floo Network Office, which is part of the Department of Magical Transportation in the Ministry of Magic, and she policed the Hogwarts fires, except for Umbridge's, to make sure that nobody in the school made contact with anyone the Ministry did not want them to have contact with.

As Cho had a romantic interest in Harry Potter, who was at the centre of the Ministry's smear campaign, this left Marietta in a very terrible position. Marietta joined Dumbledore's Army entirely because of peer pressure. Dumbledore's Army was doing the one thing the Ministry didn't want to happen, that was training students in combat.

Eventually, Marietta betrayed the D.A. to Umbridge, however, the parchment that all members had signed had been jinxed by Hermione, so she broke out with pustules which spelled the word, "SNEAK". Umbridge took Marietta to testify against Harry, but Kingsley Shacklebolt swiftly modified her memory, to keep Harry and his friends out of trouble.

This incident caused a rift in Harry and Cho's relationship. Harry blamed Marietta, though not Cho, for the results of her betrayal, but Cho insisted that was it hard for Marietta having a mother in the Ministry and blamed Hermione for jinxing the parchment. It is never explained what happened to Marietta after she betrayed the D.A.