The Transcendence actor joked about declaring his departure from acting but said Letterman's imminent sign-off stole his thunder.
Instead, Depp gushed about his engagement to actress Amber Heard and the unconventional ring he was wearing.
"I have a female engagement ring," Depp shared. "It was too big for my girl."
The stunning rock turned out to be a perfect fit for the 50-year-old actor. Depp recently confirmed his engagement to Heard, 27, but the betrothed have yet to dish on wedding details.
Earlier in the show, Letterman shocked audiences by announcing his retirement from late night. After 32 years of hosting and 21 years at CBS, Letterman said he will step down from his desk duties in 2015.