"In Your Letter" is a song written by Gary Richrath that was first released on REO Speedwagon's 1981 album Hi Infidelity. It was released as the fourth single from the album and just made the Top 20 on the Billboard Hot 100 chart, peaking at #20. It also reached #26 on the Billboard Adult Contemporary chart. It also had some chart success in Canada, reaching #34.

Richrath was inspired to write the song based on a real life incident. According to band member Kevin Cronin, at the end of a tour fellow band member Neal Doughty came home to find a on the kitchen table letter from his wife informing him that she had left him for another man. The other man turned out to be the person who supplied the band with their "illegal substances." According to Cronin, Doughty's response to the letter was "I’m really gonna miss that guy." Casandra Armour of vintagerock.com says that lyrics contain "cutting accusations with cruel alliteration like 'But you hid behind your poison pen and his pride' and 'You could have left him only for an evening let him be lonely.'""

The music of "In Your Letter" is a throwback to songs of the 1950s and 1960s. Allmusic critic Stephen Thomas Erlewine describes it as a "sun-kissed '60s homage." Philippa Hawker of The Age describes it as "a grimly uptempo 60s style whinge." Leslie Michele Derrough of Glide Magazine describes it as sounding like a "1950s sock hop." Gerald Martineez described the song as "an uptempo tune about an angry lover complaining of the way he was dumped." Armour described the music as having a "dizzying doo-wop feel." Hawkins also noted a stylistic similarity with the Pointer Sisters' song "Should I Do It" from their 1981 Black & White album. In a demo version released on the 30th anniversary version of Hi Infidelity the guitar part has some rockabilly feel.

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