A teenager told the jury she did not at first believe her friend's claims she had had a sexual encounter with the former Sunderland and England star, who has admitted sexual activity with the 15-year-old girl in his black Range Rover.
But the girl told Bradford Crown Court her friend then showed her '20 times' messages on her phone from the 28-year-old father of one.
The girl, who cannot be identified because of her age said: "At first I thought, 'It's definitely not Adam Johnson, then ..."
Kate Blackwell QC, prosecuting, asked the witness, giving evidence via a video link: "Why did you not think it was Adam Johnson?"
The girl in a strong north eastern accent replied: "Cos he's, however old he is. And why would he? He's a bit of a paedophile if he's going for a 15 ... kids and that."
Johnson has pleaded guilty to grooming the girl aged 15, between 30 December, 2014 and 26 February, 2015, and sexual activity with her, involving kissing on 30 January, last year.
But the footballer, who has been capped 12 times for his country, is on trial after denying two further counts of sexual activity with a child.
It is alleged at a second encounter in his car he penetrated her with his finger and she gave him oral sex.
The youngster, a football fan, had an 'enormous crush on him' but he 'abused his position in society' after the two began exchanging messages on social media, it is alleged.
They initially met up so he could personally sign football shirts for the girl, the court heard.
After this first meeting the two joked in their messages about whether Johnson deserved a 'thank you kiss' and they met again, when the sexual activity is alleged to have taken place.
The witness agreed that the messages between her friend and Johnson could be described as 'flirting', adding: "He must have known she was not 16. He had said 'Oh well, when are you going to be 16?' And he said 'Oh, that's ages away'."
The court heard that Johnson and the girl began communicating on Facebook, Instagram and WhatsApp, and the footballer allegedly set up a second Snapchat account so his girlfriend would not discover his activities.
But after the girl showed her friends the messages and discussed it with them, the news spread like wildfire on Facebook and around her school.
She eventually told her parents and police were called in.
A second friend of the girl also gave evidence.
She said the girl told her while they were in the school dinner hall of her second meeting with Johnson in his car and alleged sexual activity, which Johnson denies, which began with 'kissing with tongues'.
The witness added: "She told me she sucked him off, which is oral sex I believe, and he fingered her three times. It was over in half an hour, I think she said."
The witness said her friend told her a third encounter was arranged after Johnson texted her while she was at school, asking: "Are you doing anything tonight?"
"She told us that she was going to have sex and sleep with him in his car," the witness told the jury.
But when her friend turned up, Johnson was not there and he messaged her to say he could not meet her because he did not have an excuse to leave the house to give his girlfriend, Stacey Flounders.
Her friend was 'not very happy with him', the jury heard.
"She had come over, she was mad at the fact that he had messed her about," the witness said.
"She had come over to see Adam and then he did not show up. So she was mad at him.
"That was the same night she told her mam and dad."
The jury heard that that night boys in the girls' year at school found out after the girls 'let it out', and the news was on a group chat on Facebook.
The witness said her friend was 'pretty distraught' and crying because the news was out.
Earlier, the jury of eight women and four men were told of social media exchanges between Johnson and the girl, who had made it clear she was 15, after their encounter in his car where the footballer wrote: "It was class. Just wanted to get your jeans off, LOL."
The court heard the girl replied: "Next time."
The girl's messages also congratulated Johnson on the birth of his daughter and Johnson was messaging the girl and his partner simultaneously after the sexual activity had taken place.
The pair first met on 17 January last year, near a Chinese takeaway close to the girl's home and he signed two shirts for her.
Johnson later researched 'legal of age of consent' on the internet.
He also allegedly asked the girl to delete messages between them.