In the 16th century, the name La Montaña (The Mountain) was widespread in popular usage and in literature, as a designation of the Ancient Cantabria, as opposed to Castile, which referred solely to the Central Plateau. This distinction has survived into modern times.

With the rise of the Catholic Monarchs, the Brethren of the Marshes disappeared, leaving the Coregiment of the Four Villas, which included the whole area of influence of the old Brethren of the Four Villas (almost all of Cantabria). During the ancien régime, the greatest jurisdictional lordships of Cantabria were mainly under the control of three of the Grandee families of Spain: that of Mendoza (Dukes of Infantado, Marquises of Santillana), of Manrique de Lara (Marquises of Aguilar de Campoo, Counts of Castañeda), and to a lesser extent that of Velasco (Dukes of Frías, Constables of Castile).

From the 16th century on, there was renewed interest in studying Cantabria and the Cantabri, particularly concerning the precise location of the territory that this people had occupied. It was not until the 18th century that the debate about the location and size of Ancient Cantabria was settled in a series of works which described the history of the history of the region such as La Cantabria by the Augustinian father and historian Enrique Flórez de Setién. Concurrent with the resurgence of this interest in the Cantabrians and the clarification of the aforementioned polemic, many institutions, organizations and jurisdictions in the mountainous territory received the name of “Cantabrian” or “of Cantabria”.

In 1727, the first attempt to unify what would later become the Province of Cantabria occurred. Despite this, the high level of autonomy that the small entities of the fractured estate of Cantabria enjoyed, combined with a lack of resources, continued to be the main reason for Cantabria’s weakness, aggravated by the progressive advance of the Bourbonic centralism and its administrative efficiency. The latter continually emphasised the impossibility of the smaller territories facing a multitude of problems on their own: from communications to the exercise of justice, from putting aside adequate reserves for hard times to the indiscriminate levees for soldiers, and above all the progression of fiscal impositions. All of this led to an acceleration of contact between villas, valleys and jurisdictions, which tended to focus on the Assemblies of the Provinces of the Nine Valleys, led by the deputies elected by the traditional entities of self-government.

There were two events that triggered the culmination of the integration process in this second attempt:

On the one hand, the collective interest in avoiding making contributions to the reconstruction of the bridge of Miranda de Ebro, imposed by order of the Intendant of Burgos on 11 July 1775, the same year that Cantabria suffered two tremendous floods, on 20 June and on 3 November. There was a need to face as the banditry that operated with impunity in Cantabria as a result of a lack of local juridical resources. After the General Deputy of Nine Valleys gathered the affected jurisdictions to the assembly that was to take place in Puente San Miguel on 21 March 1777, they sent their respective deputies with sufficient authority to join with the Nine Valleys.

In this General Assembly a framework was established, and formal steps began to be taken, leading to administrative and legal unity in 1778. This all culminated in the success of the Assembly held in the Assembly House of Puente San Miguel on July 28, 1778, where the Province of Cantabria was constituted. It was achieved by passing the common ordinances which had been developed to that end, and which had been discussed and approved previously in councils of all the villas, valleys and subscribed jurisdictions. They were, in addition to the Nine Valleys: Rivadedeva, Peñamellera, the Province of Liébana, Peñarrubia, Lamasón, Rionansa, the Villa of San Vicente de la Barquera, Coto de Estrada, Valdáliga, the Villa of Santillana del Mar, Lugar de Viérnoles, the Villa of Cartes and environs, the Valley of Buelna, the Valley of Cieza, the Valley of Iguña with the Villas of San Vicente and Los Llares, the Villa of Pujayo, the Villa of Pie de Concha y Bárcena, the Valley of Anievas, and the Valley of Toranzo.

Having learned lessons from the failed attempt of 1727, the first objective of the new entity was to obtain approval from King Charles III for the union of all the Cantabrian jurisdictions into one province. The royal ratification was granted on 22 November 1779.

The 28 jurisdictions that initially comprised the Province of Cantabria were clear in their intention that all the other jurisdictions that formed the Party and Baton of the Four Villas of the Coast should be included in the new province. To this end they set out the steps needed for this to happen as soon as those jurisdictions should request it. They would have to abide by the ordinances, having the same rights and duties as the founders, all on an equal footing. Thus, the following joined in quick succession: the Abbey of Santillana, the Valleys of Tudanca, Polaciones, Herrerías, Castañeda, the Villa of Torrelavega and environs, Val de San Vicente, Valle de Carriedo, Tresviso, and the Pasiegan Villas of La Vega, San Roque and San Pedro, as well as the city of Santander with its abbey.

Competition between the townships of Laredo and Santander led to the latter, having initially allowed the name of Cantabria for the province created at the beginning of the 19th century, later retracting its consent and demanding that it bear the name of Santander, so there would be no doubt as to which was the capital. When in 1821 the Provincial Council presented before the constitutional Courts its definitive plan for the provincial borders and legal entities, it proposed the name of Province of Cantabria, to which the Township of Santander replied that “this province must retain the name of Santander”. However, many newspapers still showed in their headings the name of Cantabria, or Cantabrian.