Schwimmer will play a college professor's son, who is traumatised after witnessing a "romantic encounter" between his dad and the main character Fiona (Lisa Kudrow), reports Entertainment Weekly.
Schwimmer is due to appear in four episodes.
Both Schwimmer and Kudrow starred in NBC's Friends prior to Web Therapy.
Last year, Schwimmer said that he is "forever thankful" to have appeared in Friends and called it the "best job" he ever had.
Kudrow has revealed that her Friends character Phoebe Buffay inspired her "to be more optimistic".
Schwimmer is the second Friends star to join Kudrow in Web Therapy. Courteney Cox, who played Monica in the popular sitcom, appeared in the first season.
Watch a clip of Web Therapy below: