A Starbucks employee has been fired after the barista mocked a customer who stuttered while ordering a drink in Philadelphia.

A Facebook post recounting the June 27 incident went viral on social media, prompting the coffeehouse chain to respond.

According to the post, the customer identified only as Sam had stuttered while giving his name after ordering a drink at a Starbucks outlet.

He was then mocked by the barista, who responded: "Okay, S-s-s-sam."

"When he received his coffee, he was shocked to see that his name on the cup was written as "SSSAM", which was disrespectful," said the Facebook post by Sam's friend Tan Lekwijit. The post also included a picture of the name on the cup.

Sam then wrote in to Starbucks, which replied with a "pretty standardised e-mail" apologising for how the barista spelt his name, and offered him a $5 gift voucher.

"Clearly, Starbucks missed the point. It was about how you treat people with speech impairments, not how you write names," wrote Sam's friend.

"There are many people with speech disorders who are in a worse position than my friend and struggle with self-esteem and self-confidence. Getting this kind of treatment from people, especially service employees, only scars them-and I beg Starbucks employees to have this in mind," his friend added.

A day later, Starbucks responded in the comments, saying that it has "zero tolerance for discrimination".

Later, in a follow-up comment, Starbucks added: "The partner (employee) is no longer with Starbucks."

The company told BuzzFeed News that it was a "very swift decision" to fire the employee.

"I can tell you there's no tolerance for this type of behavior. We've taken immediate, corrective action and have apologised to the customer," said a spokesman for Starbucks. "We want stores to be a welcoming place to visit."

The spokesman also told BuzzFeed News that Starbucks' vice-president of operations Camille Hymes called Sam personally to "directly" apologise to him.