Both teams in the August 17 fixture at the Weston Homes Stadium were charged with a breach of FA rule E20.1 following a clash which featured several players.
A challenge by Bluebirds midfielder Marlon Pack on Posh defender Nathan Thompson sparked the scenes just before half-time.
The FA revealed its punishment in a statement which read: “Peterborough United and Cardiff City have been fined £5,000 each after admitting a breach of FA Rule E20.1 following their EFL Championship fixture on Tuesday 17 August 2021.
“Both clubs admitted that they failed to ensure their players conducted themselves in an orderly fashion during the 43rd minute of the fixture.”
The game finished 2-2, with Cardiff fighting back from two goals down thanks to a late brace from Aden Flint.