Several of the club's Under-23s reportedly clashed with a group including friends of academy manager Steve Harper's son, according to the Daily Mail.
It is understood James Harper was not involved but tended to injuries after the men traded punches at Tup Tup Palace in the city centre.
Summer signing Charlie Wiggett is thought to be one of the players left with facial wounds.
Now, The Sun reports those involved have been reminded of their responsibilities by the club after receiving a stern word from Harper.
Police were not called to the incident, but the Magpies have reportedly launched an internal investigation after Harper was informed.
The former goalkeeper is a Newcastle legend after 20 years at the club and recently took over the head academy role following the departure of Joe Joyce in the summer.
Trouble off the field is the last thing Newcastle need following a dreadful start to the Premier League season and mounting pressure on manager Steve Bruce.
The U23s are also going through a difficult time, currently without a head coach after Chris Hogg left for MK Dons and Gary Caldwell spent just three weeks as interim boss.