"They both went into this relationship without any expectations or pressure -- it was just fun and easy -- but things quickly got serious," the source says.
The couple was photographed out and about in New York City on Thursday, holding hands and smiling as they walked through the city. ET's source notes that the trip to the Big Apple is a fun getaway for the couple.
"Justin used to live in New York and is loving taking Kate to some of his old favorite places in the East Village," the source says.
Long, 43, and Bosworth, 39, have been rumored to be in a relationship since last year, when they worked together on the film House of Darkness. Last August, Bosworth officially announced that she and Michael Polish, her husband of nearly eight years, were calling it quits.
Then, in December, Long referred to his "girlfriend" during an episode of his podcast, Life Is Short With Justin Long, though he offered no other details about her identity or their relationship.
The next month, Long publicly supported Bosworth by posting about the Sundance Film Festival premiere of her flick, Bring on the Dancing Horses, on his Instagram Story.
In April, the pair was photographed together departing from a Santa Monica, California, dinner date. That same month, Long appeared on The Viall Files podcast and gushed about his girlfriend, though he didn't mention Bosworth by name.
Before getting into a relationship, Long said he'd "gotten to a place where I was comfortable with myself." While he "didn't know it at the time," Long said he was "ready for the one," whom he's now "found."
"I want to [talk about it], but I also want to be protective," Long said of his relationship. "I want to scream it from the rooftops, but I also want to be protective. It's sacred."
Later, in April, Long and Bosworth were photographed arm in arm and, at one point, mid-liplock as they lounged in the sand during a trip to Hawaii.