He said: "I'm ashamed I did that. I knew I should not be doing that. It was because I was in my hotel room - I was bored."
Speaking about his second meeting with the girl, Johnson said he knew there was a "possibility" of touching the girl sexually and it was something he wanted.
He told the court: "That was stupidity from me and I really regret that."
Johnson said they chatted about football before the girl said she had come for her "thank you kiss". He said he wanted to kiss the girl and enjoyed it.
He said: "I had all sorts of things going on in my head at the time. I was thinking: 'What am I doing? I've got so much to lose'."
The jury were told the footballer was initially befriended by the girl on Facebook before getting her phone number and using messaging service WhatsApp to chat with her.
In one WhatsApp message, Johnson said: "Need to keep it between us and delete our messages after we have spoken. Don't want anyone reading them just to be safe."
Johnson also admitted sending messages to other women, who were in their 20s, around the time his girlfriend gave birth in January last year.
Asked how the messages to other women compared with those to the girl, Johnson said: "They were different. They were a lot more explicit."
He told the jury: "I wasn't being a good person to my girlfriend or my daughter. I was talking to people I shouldn't have."
He said of the girl: "I was attracted to her."
Johnson arrived at Bradford Crown Court without his girlfriend Stacey Flounders today. She was last at court last Tuesday.
At the start of Johnson's evidence today, his QC Orlando Pownall asked him: "'You pleaded guilty to sexual activity with a child and meeting a child following sexual grooming. Did you accept kissing [the gir])?"
Johnson replied: "Yes."
Mr Pownall continued: "Did you interfere with her or indulge in oral sex with her?"
The footballer answered: "No, I never."
The court has previously heard that Johnson searched "legal age of consent" online just days after he met the girl.
He said today that the search was nothing to do with meeting the 15-year-old and was instead the result of a changing room conversation with his teammates.
He claimed the players were discussing a TV programme and the "foreign lads" didn't know the law.
Johnson told the court: "We were having a discussion in the dressing room about a programme that was on a couple of days ago about different ages and things like that.
"There are a lot of players in our team from all sorts of countries and we were having a discussion about laws and ages and things in different parts of the world."
He said he knew kissing the girl was a criminal offence, but said he did not realise that communicating with her with the intention of kissing her prior to meeting was also an offence.
The court heard that Johnson accepted the girl as a friend on Facebook but declined her request to follow him on Instagram. They exchanged telephone numbers and swapped messages on WhatsApp.
Mr Pownall asked: "Were you contemplating any kind of sexual encounter with her?"
Johnson replied: "No."
He said he asked for the teenager's number because she had been asking for signed shirts.
When asked why he told her not to tell anyone she was messaging him, he said: "Loads of reasons - I had a girlfriend, I knew what could be made of speaking to a girl that age, I didn't want her to tell her friends and them to come and ask me for shirts as well."
Johnson said being a professional football was "every young boy's dream" but admitted it made players arrogant because "you get everything done for you".
The court was also read a statement Johnson made to police after his arrest in March last year.
In it, he says: "I recognise my behaviour was wholly unacceptable. I accept the stupidity of my actions.
"I wholeheartedly apologise to [the girl]. She is a child and ought to have been safe in my company."
In the statement, Johnson admitted kissing the girl "fully on the lips" during the meeting in his Range Rover on January 30 last year but denied more serious allegations.
Prosecutor Daniel Thomas said Johnson told police he had "no specific recollection" of being sexually aroused but admitted it was a "possibility".
He said his initial meetings with the girl were "platonic in nature" and she would regularly approach him and request signed shirts.
It also emerged today that police officers investigating Johnson met bosses of Sunderland AFC shortly after detaining the star.
Detective Inspector Aelfwynn Sampson, the Durham Police officer who led the investigation, told the trial today that police were contacted by the club soon after they went to Johnson's mansion early last year.
He said: "Myself and a colleague went to meet [chief executive] Margaret Byrne to discuss the offences and consider safeguarding for any other young people he may come in contact with through his work as a professional football player."
The court heard last week that Johnson twice met a 15-year-old in January last year after she befriended him on Facebook.
At the first meeting, the girl got into Johnson's Range Rover Sport, where he signed two football shirts for the avid football fan.
The pair then exchanged scores of messages online, at which Johnson spoke about getting a "thank you kiss" from the girl, the jury heard.
When he met the teenager in his car for a second time, she says she performed a sex act on him and he put his hand down her trousers.
The court heard that, in an exchange of messages after the second meeting, he wrote: "Just wanted to get your jeans off, LOL."
The girl told some of her friends about her meetings with Johnson and rumours about them soon spread around her school, the court heard.
She broke down after other pupils chatted about her on Facebook and she told her parents, who took her to the police, the jury were told.
In his police interview, Johnson denied any sexual activity with the teenage girl other than kissing.
The court heard the girl only made the more serious sex claims against Johnson in a second police interview, and did not mention them in her first meeting with officers. She told the jury she was trying to protect the star.
Friends of the alleged victim have told the court she is "someone who can exaggerate" and took a photo outside the player's home when he pulled out of a third meeting.
The court also heard that the girl had posted messages on a social media account of another Sunderland star, Connor Wickham, who passed them on to police.
