The New Wales Link Path spans an extra 18 miles (29km), joining the route near the Cheshire border at Saltney Ferry.
It takes in four counties in all, stretching into Wrexham at Caergwrle and meeting the Offa's Dyke trail at Llandegla Forest in Denbighshire.
Flintshire council secured Â£30,000 in funding to pay for route-markers.
"If you are looking for a bit more of a challenge to walk around Wales, this route is for you," said councillor Bernie Attridge, cabinet member for the environment.
Work on Wales Coast Path, the world's first round an entire country, began in 2007 with the formal opening in May 2012.
A new link has been added to the 870-mile Wales Coast Path at Flintshire for walkers seeking a "bit more of a challenge".