The 35-year-old actor stepped out with the 37-year-old actress to the movies Monday in West Hollywood after filing for divorce from wife Aryn Drake-Lee in April, according to TMZ.
"They really care about each other and are dating exclusively," a source told E! News. "Minka has been there for Jesse not only as a girlfriend, but also as a friend."
"Jesse is really private with everything, especially his relationships, but feels open with Minka and she has been supportive and that's what he needs now," the insider added.
"They're having a good time together," a source told the magazine.
Williams, who shares daughter Sadie and son Maceo with Drake-Lee, filed for divorce April 11 after more than four years of marriage. He addressed the split and cheating rumors in Jay Z's short film Footnotes for 4:44, which was released Monday.
"I was in a relationship 13 years, 13 real years, not 5 years, not 7 years -- 13 years. All of a sudden [expletive] are writing think-pieces that I somehow threw a 13-year relationship," the actor said.
"Like, the most painful experience I've had in my life with a person I've loved with all of my heart -- that I threw a person and my family in the trash because a girl I work with is cute," he added of the speculation.
Williams plays Dr. Jackson Avery on the ABC series Grey's Anatomy. Kelly is known for portraying Lyla Garrity on Friday Night Lights and Valerie Stahl on Almost Human, and recently appeared on Jane the Virgin.