The acting legend, who plays Rita Tanner in the ITV soap, hits the big milestone on September 30, but the special won't air on the channel until October 13, in a primetime Friday 9pm slot.
The special will be hosted by The Chase and Doctor Who star Bradley Walsh and feature a whole host of Corrie actors past and present gathering to celebrate Knox's career and reminisce over the most memorable moments of her life.
ITV's Head of Entertainment Commissioning Katie Rawcliffe said when the special was announced that: "It's easy to throw the words 'legend' and 'icon' around, but Barbara Knox really does embody both of those terms.
"It's a privilege to tell the story of one of British television's most enduring stars as she celebrates a very special birthday."
Knox first appeared on Corrie in 1964 and returned in 1972, and is the second-longest-serving cast member behind Ken Barlow actor William Roache.
Meanwhile, another Corrie star has confirmed that he will be returning to the soap after taking a break.
Jack Webster actor Kyran Bowes said on social media that: "I took some time away to concentrate on my studies. Happy to say I'm back and even happier to say I passed my GCSES. Hard work and perseverance really does pay off."
Coronation Street airs on Mondays, Wednesdays and Fridays at 8pm on ITV1 and streams on ITVX. Barbara Knox's 90th birthday special airs on Friday, October 13 at 9pm.
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