Ian Holt worked at the Redditch delivery office
It follows an industrial tribunal ruling that a white postman was victimised and unfairly dismissed after supporting his black colleagues.
Ian Holt was acting manager at Redditch delivery office when he witnessed racially-abusive comments being made against three black postmen in 1998.
He was due to give evidence in support of them at a tribunal.
Claims rejected
But a month after agreeing to speak out, Mr Holt was sacked by the then office manager, Eddie Marriot.
The tribunal ruling in favour of Mr Holt was announced on Monday.
It rejected Mr Marriot's claim that the dismissal was due to wilful delaying of mail.
It ruled that Mr Holt, who had returned to his psotman duties at the time of his sacking, had been victimised under the race relations act and unfairly dismissed.
In a statement, Royal Mail said it was disappointed by the tribunal ruling but added there was no place for harassment or victimisation within the Royal Mail.
It announced that an internal inquiry had been started and said that it would take any appropriate action necessary once the inquiry was complete.
Andrew Holt, of the Worcestershire Racial Equality Council, said: "It's a very welcome reaction but frankly the post office could have taken exactly the same course as the tribunal - investigated, come to the same conclusion - years ago.
"It is very sad the post office tried to defend the indefensible."