Two officers and the dog, called Ernie, had been chasing a suspect in West Drayton on Tuesday, following reports of an assault and theft.
A 28-year-old man has been arrested on suspicion of robbery, resisting arrest, common assault, and animal cruelty, the force said.
The suspect needed hospital treatment for a bite injury.
He has since been released under investigation while inquiries continue.
Ernie was taken for assessment and sustained no major injuries. "He has returned to normal duties," a police spokesperson added.
Supt Emma Richards said: "Violence against anyone - whether it is a member of the public, a police officer or police animal - is completely unacceptable and this should send a clear message that we will take such acts of violence seriously."
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