The research scientist, 35, posted a video of himself wearing a traditional floral garland (mala) around his neck on Instagram.
In the caption, he told his more than 397,000 followers that he and his soon-to-be wife are “very excited and a little nervous” about the big event.
Mandal wrote: “I have something to announce. I am having my social wedding tomorrow. We both are very excited, and a little nervous. It will be a small wedding in India. And yes, to everyone who guessed that this time I am in India for my wedding.
“I know you all will have a lot of questions. And me and my wife will answer them all. If you want to know anything about her, we will also be very happy to answer it too.
“But most of all, we would love to have your blessings and good wishes. Over the past few years, my Instagram family always been [sic] a huge support for myself. You all make me grow in confidence and be myself. In Bengali culture , we love to invite all our family and friends in our wedding.”
He invited his followers to attend his wedding “emotionally (and also maybe virtually)”, adding that it “means a lot”.
Friends and fans flooded his comments with well wishes and messages of congratulations, including several Bake Off alumni.
Ruby Bhogal, who was a finalist in 2018, wrote: “I’m so so so happy for you both – so sad I can’t be there to celebrate with you but we’ll be sure to carry on the party when you return. Proud of you Rahul, wishing you all the happiness in the world.”
Karen Wright, who was also a contestant in the same year, added: “Wonderful news Rahul, wishing you both every happiness.”
Giuseppe Dell-Anno, who won the 2021 series of Bake Off, also left a comment and said: “Congratulations Rahul! I’m over the moon for you! Enjoy this exceptional time and savour it all with your friends and family!”
Mandal also shared clips taken during the preparation for the ceremony, showing himself and his family putting up decorations and wearing matching bright orange outfits.
He moved to Rotherham, South Yorkshire, in 2015 from India and only began baking then to make friends with people who worked at his local supermarket and leisure centre.
In a recent interview with The Herald Scotland, Mandal said: “Growing up, I never had a lot of friends, but I had a huge family around me. And in a strange way, the people in Rotherham kind of became part of myself.”
After he won Bake Off, judge Prue Leith said of Mandal: “I think the amazing thing about Rahul and why I am so proud of him is that he came in unconfident, but he just kept going.”