Shentall (centre) used to be in pop group Boom!
He will take the place of Kym Marsh, who left the band last month.
Shentall was the "cream of the crop" in the weekend's auditions, his new bandmates said on Tuesday.
Shentall told a press conference in London he was a fan of the band, saying: "I think they are great and I think they make people smile."
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The other members - who Shentall will now join on recordings and a national arena tour later this year - said he stood out from the others who auditioned.
"It was really hard to make the decision as there was so much great talent but Johnny is perfect for Hear'Say," Suzanne Shaw said.
"He has a fantastic voice and shares our sense of humour, which will help him fit in quickly."
Noel Sullivan billed the band's future as: "Hear'Say - the new chapter."
Myleene Klass said they had not known whether they wanted a male or a female replacement for Marsh, but they had "let talent speak for itself".
While with Boom!, Shentall enjoyed a number 11 UK hit single while with Falling, and took part in TV appearances and a Smash Hits tour that was seen by thousands of pop fans.
Click here to watch Boom! perform Falling on Top of the Pops
He first met Scott-Lee when he auditioned to become a dancer with Steps, and proposed on a boat on the River Thames on Valentine's Day 2001.
Shentall said his future wife did not mind that he was about to join Hear'Say, despite the fact that she has just left the pop limelight after Steps split up.
"She's very happy and she's very proud and she can't wait to see me," he said.
But he declined to show off his singing voice at the announcement.
He will now go straight into the band to begin recording, and his progress will be shown as a TV series - much as the selection process of the original band members was.
One of the finalists on the ITV1 show Pop Idol, Korben, also made it through to the group of 12 in the final stages of the auditions - but was not chosen.
Kym Marsh left the group last month, amid a flurry of media speculation that she had fallen out with fellow member Myleene Klass.
Even with the added momentum of a new member, the band faces a challenging time as their popularity had been on the wane and many had predicted that they would not last until the end of the year.
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