"The Daily Show"/Comedy Central
Vulture first reported the news, though CBS wouldn't confirm Stewart's appearance. It's also unclear what Stewart will do during the convention coverage and whether it will be ongoing through the convention, a simple cameo, or anything in between.
Colbert is set to air live at 11:30 p.m. for the two weeks of convention happenings. He'll host from the New York studio, with a team on the ground in Cleveland and Philadelphia for the GOP and Democratic conventions, respectively.
This will be only the second late-show appearance for Stewart this year. Previously, he made a cameo on TBS's "Full Frontal with Samantha Bee" last month.
There's personal gain in Stewart appearing on the "Late Show," since he's one of its executive producers.
Others joining Colbert during the convention coverage include "Last Week Tonight" host and fellow "Daily Show" alum John Oliver, comedian Keegan Michael-Key, comedian Bassem Youssef, Senator (and potentially Hillary Clinton's running mate) Elizabeth Warren, among others.