Rivers, 81, at the time of her death in September, never won an Oscar, but was a staple on the red carpet.
She also appeared in films including "The Muppets Take Manhattan", "Spaceballs", and "The Smurfs".
People watching the Oscars were outraged by the perceived snub and took to Twitter to vent their anger.
They also took issue with the omission of comedienne Elaine Stritch.
"Dear @TheAcademy: It was really shameful that you didn't include #JoanRivers in your 'In Memoriam' tribute at the #Oscars. That was wrong," an user wrote.
"Omitting Joan Rivers from that in memoriam was genuinely disrespectful," wrote Melissa Radz.
In addition to the usual montage of pictures and names, the Oscars In Memoriam segment included a touching introduction from Meryl Streep, as well as a performance by Jennifer Hudson, who sang "I Can't Let Go" from Smash.