The Mad Men star will appear in five instalments of the comedy's six-episode second season, the show's network IFC announced.
However, details of Hamm's character in the show have not yet been released.
Hamm is best known for his role as Don Draper in Mad Men. He has also had guest roles in comedies like 30 Rock and Children's Hospital and recently starred in the movie Bridesmaids.
Meanwhile, The Smiths member Johnny Marr, who worked on the first season of Todd Margaret, will return to provide more original music for the soundtrack.
The Increasingly Poor Decisions of Todd Margaret stars David Cross as Todd Margaret, a man who ends up landing a senior sales position in London.
Sharon Horgan plays Alice, the object of his affection, while Will Arnett appears in the show as Todd's boss Brent.
Cross has previously claimed that the show was "a little overly influenced by Lost".
The Increasingly Poor Decisions of Todd Margaret will return to IFC next year.
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