Matthew Spry, now 28, of Boston Road, Sleaford, admitted charges of sexual activity with a girl under 16 and causing a child to watch a sexual act. The charges both related to the period between May 1 and September 17, 2019.
He was jailed for three years and four months and given a sexual harm prevention order.
Spry initially met the girl while he was working at a fair and later contacted her via Snapchat before meeting her.
Victoria Rose, prosecuting, said that Spry had sex with the girl even though he knew she was only 15.
Spry also sent her a video over Facetime which showed himself masturbating.
The girl, who was described as vulnerable, screen-recorded the footage onto her phone and later showed it to a support worker who contacted police.
The court was told that when the girl’s mother discovered that Spry had contacted her daughter she warned him to stay away.
Miss Rose said: “That led to the arrest and interview of the defendant. He accepted that sexual intercourse had taken place but at first said he believed she was 16.
“Later he accepted he had sent the video and that he knew she was 15.”
Judge John Pini QC, passing sentence, told Spry: “You were a fully grown adult when you committed these offences. You were the adult and she was a child. You knew, or very quickly came to know, that she was under age.”
Karen Walton, in mitigation, said that Spry had admitted what he did and had no previous convictions for sex offences.