The former BBC Radio 2 presenter will host a programme from 9am to midday on Classic FM, starting on January 7, 2012. He will replace Mark Forrest's Classic FM chart, which moves to Sunday afternoon.
Classic FM said that Titchmarsh will play "the finest selection of classical music, including a few of his own favourites, providing the perfect start to the weekend".
Titchmarsh, who left Radio 2 in August after the station axed his long-running Melodies For You programme, told the Classic FM website: "I am delighted to be joining the radio station that I tune in to most of all, and look forward to sharing my passion for a wide range of classical music at what to me is a very special time in the week - Saturday morning."
His arrival at Classic FM is part of a revamp of the weekend schedule to mark the station's 20th anniversary next year, including a new show called Night at the Movies on Saturday evenings.
Presented by Classic FM's movies expert Howard Goodall, the show will play soundtracks from the latest movies and classical favourites featured in films from 5pm to 7pm.
From January, Alex James's The A-Z of Classic FM Music will run on Saturday nights from 7pm to 9pm, followed by David Mellor's New CD Show.
Sunday will welcome Natalie Wheen's Great Performers, featuring celebrations of some of the legendary classical musicians from 9pm to 10pm.
Elsewhere, Anne-Marie Minhall's Sunday show will start at the earlier time of 3pm, followed by Forrest's Classic FM chart at 5pm and David Mellor's Sunday show at 7pm.