An arbitration tribunal earlier this month found in Keegan's favour and have now ordered Newcastle to pay costs and interest on the £2million compensation award dating back to September last year.
The tribunal also said the club had made "entirely unfounded allegations" against Keegan and rejected Newcastle's claims that they should not be liable for costs because they had offered to settle the case before the hearing.
A statement from the tribunal said: "We direct that the club should pay all of Mr Keegan's costs.
"Moreover, we reject the club's attempt to rely on the various offers which it made both prior to and during the hearing to settle the claim since none of them involved the club's acceptance that he had been constructively dismissed and all of them involved a condition of secrecy or confidentiality.
"We direct that the club should pay Mr Keegan's costs on an indemnity basis given the manner in which it conducted this litigation: its defence on the primary liability issue was, in our view, wholly without merit and it chose to make entirely unfounded allegations against Mr Keegan."