The 25-year-old appeared at The Magistrates’ Court on Tuesday accused of kicking Rachael Wall.
Flanagan was charged after an incident at Liverpool city centre on 22 December. He will be sentenced on 17 January.
Liverpool-born Flanagan has made more than 50 appearances for the Premier League club since making his debut aged 18.
He has one England cap and has played only one match for Liverpool this season after a long lay-off due to injury.
According to the BBC, the court was told the full-back had been seen on CCTV assaulting his partner, who he has been in a relationship with for 18 months.
Lionel Greig, mitigating, said the victim had not provided a statement and there had been no previous instances of domestic violence in their relationship.
Both parties were under the influence of alcohol at the time, he added.