The 65-year-old TV presenter — who previously worked with Reid on BBC Breakfast — will join her to co-anchor the ITV morning show from Monday 24 May until Wednesday 26 May.
Turnbull said: “I am very pleased to be back presenting on GMB, especially as I get to be reunited with my old pal Susanna.”
Reid, 50, said: “I am delighted Bill is back in the presenter chair at the GMB desk. He has been a beloved favourite of breakfast television viewers for years and a good friend of mine.”
Turnbull — who revealed in 2018 he had been diagnosed with incurable prostate cancer — previously co-hosted the show with Reid last year when Morgan, 56, was on holiday.
In November 2019, Turnbull had appeared as a guest on the programme when Reid asked him if he would return when Morgan "has been incapacitated".
Morgan quit the ITV breakfast show in March over comments he made about Meghan Markle's interview with Oprah Winfrey.
GMB is yet to announce a full time replacement for him, with Reid being joined on her days hosting the show by GMB staff members Adil Ray and Ben Shephard, former Labour spin-doctor Alastair Campbell and ex-This Morning host Richard Madeley.
Watch: Piers Morgan clears GMB dressing room and leaves Susanna Reid cut-out behind
Neil Thompson, Good Morning Britain's editor said: “I'm thrilled that Bill is rejoining us behind the GMB desk.
"It's not just his brilliant and enduring onscreen partnership with Susanna that makes its mark, it's his relationship with the audience in general, one based on authenticity, authority, warmth, humour and trust.”
Reid and Turnbull worked together on BBC Breakfast between 2006 and 2014 before she left to join the newly launched GMB. Turnbull presented his last episode of the BBC morning show with Louise Minchin in February 2016.
Watch: The awkward moment Susanna Reid rejected Adil Ray's hug live on 'GMB'