The new Springfield resident will appear in an early 2010 episode featuring Chris Martin of Coldplay as guest star.
The contest, part of the Fox show's 20th-anniversary celebration, is for those 18 and older. It opens Friday and runs until 11:59 p.m. EDT Oct. 24.
Online submissions at thesimpsons.com should be "pithy and funny," said executive producer Al Jean, who will help judge them along with series creator Matt Groening.
The winner gets a trip to Los Angeles to work with producers and the show's animation director. The top entry will be announced in mid-November.
Entries are expected to describe the character's traits, including his or her name or nickname; age; appearance; occupation and catch phrases (think Homer Simpson and "D'oh!").
"It's gotta be a human being. We've narrowed it down that much," Jean said.
The proposed character should have a typical "Simpsons" overbite and be yellow or another hue found in the Springfield universe, he said.
The contest is a tip of the hat to fans, Jean said.
"We can't thank them enough," he said. "We're now in uncharted waters: We've gone longer than any scripted show in prime-time history. It's a cliche, but it wouldn't be so except for these people who've loved the show all these years."
The record for the longest-running prime-time series had been held by "Gunsmoke," the Western drama that aired on CBS for two decades. "The Simpsons," which aired its first regular episode in January 1990 after a 1989 Christmas special, has been renewed through the 2010-11 season.
The contest's winning character, to be drawn by series animators, has a reserved spot in the episode in which Homer disappoints wife Marge by arriving late at a wedding after stopping to buy a lottery ticket.
He encounters the new face while rushing to the ceremony, Jean said, adding that it's possible the character could pop up on the series again.
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