The Act of Seclusion is a secret annex in the Treaty of Westminster (1654) between the United Provinces and the Commonwealth of England in which William III, Prince of Orange, was excluded from the office of Stadtholder. The First Stadtholderless Period was heralded in January 1651 by the regents, such as the republican-minded brothers Cornelis and Andries de Graeff and their Cousins Andries and Cornelis Bicker, during the Grote Vergadering in The Hague, a meeting of representatives of the States of each of the United Provinces. This meeting was convened after the death of stadtholder William II on November 6, 1650, when the States of Holland decided to leave the office of Stadtholder vacant in their province.

Through the Treaty of Westminster Johan de Witt, Grand Pensionary of Holland, not only ended the First Anglo–Dutch War, but also ensured that the orangist faction would no longer hinder the regents, so that English Republicans no longer needed to fear that William III (three years of age at the time) could become a strong Dutch leader who could bring the throne back to the Stuarts to whom he was closely related. Ironically, William III would later drive out the Stuart King James II during the Glorious Revolution and thereby end moves in England towards absolutism.

As the other provinces would have refused to sign the treaty if they had known of the secret clause, de Witt arranged that this clause would only be signed by the States of Holland. The States-General of the Netherlands were completely left in the dark, as was the Frisian plenipotentiary at the negotiations. Only the two Holland representatives (one of whom was the renowned diplomat Hieronymus van Beverningh) were in on the secret. Consequently, the States-General ratified the treaty on April 22, 1654, without the secret annex. The prime movers behind the Act of Seclusion, in which William III, Prince of Orange was excluded from the office of Stadtholder, was Jan de Witt and his uncle Cornelis de Graeff. Then the States of Holland debated the Act and passed it on May 4, 1654, over the opposition of the Holland ridderschap and six of the cities. Only then did the English Parliament ratify the treaty (including the secret clause) as agreed beforehandIsrael, The Dutch Republic: Its Rise, Greatness and Fall, 1477-1806, p. 722-723.

According to Grand Pensionary de Witt, it was Oliver Cromwell who demanded the secret annexIsrael, The Dutch Republic: Its Rise, Greatness and Fall, 1477-1806, p. 725. and he managed to have it ratified only with the greatest effort. The Deputated States of Friesland even demanded that the conduct of the Dutch plenipotentiaries be investigated. When the Act of Seclusion became known later, it was commonly assumed that de Witt masterminded it himself. In the 19th century, investigation of his secret correspondence appeared to acquit him of this. Nowadays, different positions are taken in this matter stemming from the insight that DeWitt may have manipulated these writings out of fear that they may fall into the wrong hands. He was seldomly spontaneous when writing or speaking.

When the English Restoration brought Charles II to the throne of England, the States of Holland declared that the Act of Seclusion had thereby lost its validity, since the Act was signed by the Commonwealth which had now ceased to exist.

In 1667, de Witt and his partisans tried to permanently bar the House of Orange from influence by the Perpetual Edict. However in 1672, the States revoked the Edict and made William of Orange Stadtholder.