The specialist contractor has formed a joint venture with project developer LRCH to invest £25m into the London Resort Project near Ebbsfleet.
The £5bn scheme was dealt a blow last year when film studio Paramount Pictures walked away from the project.
Proposals for the park have been around since 2012 with the latest timetable promising a planning application next year.
Steve Norris, Chairman LRCH said: “We are delighted to welcome Keltbray to this most exciting project.
“We have been discussing for some time the best way to harness their specialist construction industry expertise alongside their ability to act as a catalyst for further industry participation and investment.
“Over the next five years, this project will become one of the largest construction projects in the UK and their involvement takes us a significant step closer to delivering a world class resort.”
A £5bn investment will transform seven million square feet of land into two theme parks celebrating brands from across literature, television, music, film and entertainment.
Brendan Kerr, CEO of Keltbray, said:“Keltbray is recognised in the industry as an enabler of projects.
“It has long prided itself on its ability to manage the most challenging of projects thanks to its investment in training, design and plant.
“At seven million square feet, this new theme park won’t be an easy project but that is why the formulation of this JV with LRCH is an ideal partnership.
“We’re delighted to be involved in what I’m sure will provide a significant boost to both employment and tourism in the UK.”