The move is being billed by US media as a major breakthrough for trans visibility in popular culture.

Eight-year-old Jackson Millarker, who is transgender, will guest star in an episode of the series titled A Stereotypical Day.

It is believed to be the first time a network program has featured a transgender child actor in a role.

The episode, which is the second episode of the show's eighth season, will see parents Cameron (Eric Stonestreet) and Mitchell (Jesse Tyler Ferguson) invite their daughter's friend Tom over for a play-date.

Tom is played by Millarker in the episode, which will air in the US this week.

A picture of Millarker was posted to the Instagram account of director Ryan Case, who works on the series.

It is not clear if Case worked on the episode; an episode synopsis from 20th Century Fox, which produces the series, names Beth McCarthy-Miller as the director.

"This is Jackson Millarker, he's 8-years-old, from Atlanta, and just happens to be transgender," Case wrote on Instagram.

"He plays Lily's friend Tom in this week's Modern Family and he's wonderful," Case added. "One of the many reasons I love being a part of this show."

Though a number of programs have featured transgender adults, it is believed this is the first time a scripted program has hired a transgender child to play a transgender role.

Modern Family airs in Australia on the Ten Network.