ITV will buy a 65% stake in Los Angeles-based Thinkfactory Media, with an option to acquire the remaining 35% within five years.
It adds to a raft of recent acquisitions to boost its production arm, including a controlling stake in US reality TV maker High Noon Entertainment and London-based producer The Garden, maker of fly-on-the-wall documentaries such as 24 Hours In A&E and Inside Claridge's.
Thinkfactory Media - founded in 1992 by Hollywood producer Leslie Greif - produces reality, entertainment and drama.
Its hit mini-series Hatfields & McCoys picked up a number of Emmy awards last year, with Mr Costner named outstanding lead actor.
ITV chief executive Adam Crozier said Thinkfactory Media had a "terrific reputation".
He added: "We're gathering real momentum in building a substantial US presence, which is central to our strategy of creating a strong international content business. The acquisition of Thinkfactory Media is clearly a further significant step forward in achieving that goal."
Thinkfactory will join the group's growing US arm, ITV Studios America, which also includes reality television producer Gurney Productions, which makes shows including hit US cable series Duck Dynasty featuring a family business making hunters' duck calls.
ITV bought a 61.5% shareholding in Gurney at the end of last year.