The Union Corps engaged on the first day of the Battle of Gettysburg was the Eleventh Corps.

Under the leadership of General Oliver O. Howard, this corps participated in the early skirmishes as Confederate forces launched an assault on Union positions. Although they managed to hold their ground at first, the Eleventh Corps encountered serious difficulties, especially when Confederate troops outflanked them. This led to a retreat through the town of Gettysburg, creating a precarious situation for the Union Army as they prepared to regroup for the battles that followed.