Williams, 31, is a three-time Oscar nominee who has a 6-year-old daughter from her relationship with late actor Heath Ledger. She and Ledger broke up months before his 2008 accidental prescription drug overdose.
The actress knows Segel, a cast member of TV's "How I Met Your Mother," through her best friend, actress Busy Philipps, who starred with Segel on the comedy series "Freaks & Geeks."
Segel, 32, also starred and co-wrote 2011's "The Muppets" movie, which earned an Oscar nomination for its song "Man or a Muppet."
"They are smitten and very serious," a source told UsMagazine.com of Williams, who lives in New York, and Segel, who lives in Los Angeles.
The celebrity news Web site said they have been spotted on romantic-looking dates and acting affectionately towards each other in public since last month.
Although they live on opposite sides of the country, the new couple are "trying to make it work," the insider told UsMagazine.com.
"She hasn't been this happy in a long time," the source said.