Seacrest, who already hosts "American Idol" and two radio shows, will co-anchor E!'s nightly newscast, "E! News Live," most likely with Giuliana DePandi.
He'll also host the network's red-carpet coverage, beginning with next month's Golden Globes, according to trade reports.
Seacrest will also develop shows for the cable entertainment network.
Seacrest's deal officially ends Star Jones' short run at E! as red-carpet host. Jones replaced Joan and Melissa Rivers, who jumped to the TV Guide Channel for a reported $8 million deal.
Where Seacrest will find the time for his E! gig is anyone's guess. His radio jobs encompass hosting a daily show on KIIS FM in L.A. as well as hosting the weekly "America's Top 40" countdown, which he inherited from Casey Kasem a few years back.
As The Post first reported, Seacrest also signed a deal with CNN to host several editions of "Larry King Live" in an attempt to lure a younger audience to CNN.
He also co-hosts ABC's "New Year's Rockin' Eve" with Dick Clark and is expected to eventually replace Clark, who missed last year's telecast after suffering a stroke.
And, of course, Seacrest hosts FOX's "American Idol," which kicks off its fifth edition next month and airs twice a week.
Seacrest, who's also launched his own clothing line, hosted "On Air with Ryan Seacrest," a daytime talk show which was supposed to give MTV's "Total Request Live" a run for its money but never really took off, despite a bevy of A-list stars who dropped by the show.
Much like former "TRL" host Carson Daly, who now hosts "Later with Carson Daly" on NBC, Seacrest is viewed as someone who appeals to younger viewers, the demographic most sought after by advertisers.
E! had that sort of personality when it aired a version of Howard Stern's syndicated radio show, which brought in young viewers.
E! officials would not confirm the Seacrest deal Wednesday.
Ryan Seacrest, E! News host:
Jon Stewart,"The Daily Show," Comedy Central:
Dan Patrick, "SportsCenter," ESPN:
Nancy Grace, CNN Headline News:
Robert Osborne, Turner Classic Movies host: