Joaquin Phoenix as Theodore

Scarlett Johansson as Samantha (voice)

Lynn Adrianna as Letter Writer #1

Lisa Renee Pitts as Letter Writer #2

Gabe Gomez as Letter Writer #3

Chris Pratt as Paul

Artt Butler as Text Voice (voice)

May Lindstrom as Sexy Pregnant TV Star

Rooney Mara as Catherine

Bill Hader as Chat Room Friend #2 (voice)

Kristen Wiig as SexyKitten (voice)

Brian D. Johnson as OS1 Commercial Lead

Amy Adams as Amy

Matt Letscher as Charles

Spike Jonze as Alien Child (voice)

Olivia Wilde as Blind Date

David Azar as Theodore's Divorce Attorney

Guy Lewis as Marriage Counselor

Melanie Seacat as Nice Lady

Pramod Kumar as Pizza Vendor

Evelyn Edwards as Mother Who Dated Pricks

Steve Zissis as New Sweet Boyfriend of Mother Who Dated Pricks

Dane White as Son

Nicole Grother as Daughter

James Ozasky as Catherine's Dad

Samantha Sarakanti as Mother of Newborn

Luka Jones as Lewman

Gracie Prewitt as Jocelyn (Birthday Girl)

Claudia Choi as Uncomfortable Waitress

Laura Kai Chen as Tatiana

Portia Doubleday as Surrogate Date Isabella

Soko as Isabella (voice)

Wendy Leon as Grocery Shopper

Lil Buck as Busker / Dancer

Robert Benard as Michael Wadsworth (Editor)

Lisa Cohen as Michael Wadsworth's Wife

Grant Samson as Michael Wadsworth's Associate

Brian Cox as Alan Watts (voice)

Alia Janine as Commuter (uncredited)