The two stars were first rumoured to be dating after being spotted kissing on an LA beach in July.
McCord confirmed their romance while attending Eva Minge's New York Fashion Week show on Saturday, reports the New York Post.
She told the newspaper that Purcell is indeed her "boyfriend", and added that her "parents like him".
McCord also beamed: "[I've] never been happier."
The 24-year-old McCord previously dated Twilight star Kellan Lutz for two years, but the couple ended their relationship earlier this year.
90210 kicks off its fourth season on Tuesday at 8/7c on The CW.
> Twilight's Kellan Lutz dating Home and Away star Sharni Vinson?
Watch a trailer for the new season of 90210 below: