Reading 107 FM, which has a popular music broadcast format, reaches 257,000 adult listeners across Reading and was launched in October 2002.
GMG Radio made its move after Milestone Radio Group decided to sell its stake. The remaining shareholding is owned by the Goodhead Group, which is controlled by Reading Football club chairman John Madejski.
John Myers, GMG Radio chief executive, said: "We were one of the original shareholders in Reading 107 FM and are proud of its achievement to date, so it makes sense to increase our stake to a level where we can now offer further management support.
"There are no plans to change the station's name or format. We aim instead to increase the level of local output and invest in marketing in order to grow the audience further."
Reading 107 FM joins GMG Radio's portfolio of five regional Real Radio and Smooth FM stations, which broadcast across London, south and west Wales, Scotland, Yorkshire and the north west of England,
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