Zane Alchin, 25, was charged with using a carriage service to menace, harass and cause offence.
In August last year, he posted a series of abusive and threatening comments to 25-year-old Olivia Melville from Sydney's south.
A friend of Alchin's had taken a screenshot of her Tinder profile and posted it on Facebook, where it was shared thousands of times.
After that, Ms Melville received many abusive online messages from strangers.
Alchin had previously pleaded not guilty to the charges and a hearing was expected to begin in Downing Centre Local Court today.
However, the court was told by his lawyer he had changed his plea to guilty.
His case will return to court for sentencing next month.
The case is expected to set a precedent for whether threats made on social media are punishable under existing federal law.
Ms Melville has only just spoken publicly about the abuse she suffered, for fear of provoking a further backlash.
She said she was concerned about losing her job in the performing arts and upsetting her parents, and has called on the Federal Government to fund a campaign to deter people from attacking women online.