She had joined the organisation as director of comms and campaigns on 1 October. Her employment officially finishes at the end of the month.
A Which? spokeswoman said: "Vickie Sheriff has now left Which? to pursue other career opportunities. We thank her for her contribution to Which? and wish her every success in the future."
Sheriff is considering her next step, but is understood not to have a new role yet.
Sheriff arrived as Which? was restructuring its senior team - Sarah Threadgould arrived at the same time from HSBC, as group marketing director, while previous campaigns and comms chief Alex Neill became MD of home and legal services.
Her departure follows the announcement recently of new head of news Cherie Willers. She joins next month from the Local Government Association, succeeding Energy UK-bound Abbie Sampson.
Founded in 1957 as a consumer magazine, Which?'s parent organisation The Consumers' Association has been a charity since 1987.
The organisation's executive pay figures have raised eyebrows in recent years - PRWeek sister title Third Sector reported in October that its CEO and three other executives had each earned more than £370,000 in the prior year.
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