Colback was charged with a breach of regulations introduced in 2014 which prevent players and coaches from placing bets on any football activity worldwide.
A club statement said: “Jack Colback has been fined £25,000 after he accepted an FA misconduct charge in relation to betting.
“The Newcastle United player breached FA Rule E8 in respect of a bet placed on the result, progress, conduct or any other aspect of, or occurrence in, a football match or competition on 16 March 2016.”
Colback’s offence came two days after Newcastle lost 1-0 at Leicester and four days before they drew 1-1 with his former club Sunderland as they fought desperately – and ultimately unsuccessfully – to remain in the Premier League.
However, it is understood the charge did not relate to a game which had any bearing on the 27-year-old’s club.
Colback missed Saturday’s 1-0 Sky Bet Championship defeat by Blackburn with an eye problem and it remains to be seen whether or not he returns to action in the EFL Cup quarter-final at Hull on Tuesday evening.