The Lake District and Yorkshire Dales national parks are to be extended to meet in the middle - with just the M6 motorway and West Coast Main Line railway now dividing more than 1, 750 square miles of protected landscapes.
The Yorkshire Dales will expand by almost a quarter, taking in some 161 square miles of land at its northern and western fringes - primarily from Cumbria, but also a small slither of Lancashire.
The Lake District national park will gain an extra 27 square miles, extending eastward as far as the motorway just south of the Tebay services.
"These are, and always have been, areas of superb scenery that deserved National Park status years ago."
Martin Holdgate, Friends of the Lake District
The Department of Environment, Food and Rural Affairs said the decision, which comes into effect in August 2016 and follows a recommendation by Natural England, would boost rural tourism and support local businesses.
Ms Truss said: "The Dales and the Lakes have some of our country's finest landscapes, beautiful vistas and exciting wildlife. They are part of our national identity.
"I am delighted to be able to announce this extension which will join these two unique National Parks and protect even more space for generations to come."
Martin Holdgate, president at Friends of the Lake District, which campaigned for the change, said: "Of course we are delighted that the Secretary of State has finally confirmed the designation orders for the extensions to the Lake District and Yorkshire Dales National Parks.
"These are, and always have been, areas of superb scenery that deserved National Park status years ago. Friends of the Lake District has worked for years to achieve this outcome, and we are pleased that our efforts have borne fruit but now everyone must work together to ensure we reap the benefits."
Opponents of the plans included five local councils, which primarily objected on the grounds that planning decisions would pass to the control of the National Park authority.
There were also fears that house prices could rise in the newly-protected areas, worsening a shortage of affordable housing, a new housing developers will have stricter hurdles in the park.
But Ms Truss said prices in the newly-designated area were already within the top quintile for the Lake District and higher than in the Yorkshire Dales national park, and that there was "no evidence to suppose that house prices would rise solely as a result of designation".
Andrew Sells, chairman of Natural England, said: "I very much welcome the Secretary of State’s decision to extend the Yorkshire Dales and Lake District National Parks in line with the recommendations made by Natural England.
"The Chancellor’s Northern Powerhouse depends upon the ‘natural capital’ provided by our iconic National Parks. They represent some of England’s most treasured natural assets.
"With international appeal, their stunning landscapes stand out as a beacon to the people who come to enjoy them whilst their intrinsic value drives the communities, businesses and biodiversity they support. Extending these today confirms their great importance and provides them with the recognition they richly deserve."