Act of Tynwald securing the customary tenants of the Isle of Man in estates of inheritance, passed in 1703 and proclaimed on the Tynwald Hill on 6 June 1704.

Its full title is an Act for the perfect settling and confirmation of the estates, tenures, fines, rents, suits and services of the tenants of James, Earl of Derby, within the Isle of Man. It was passed at a Tynwald Court at St John's Chapel on 4 February 1703, by the Earl as Lord of the Isle, Robert Mawdesley as Governor, the Lord's other officers and the twenty-four Keys.

The dispute it settled was over a hundred years old. The tenants claimed an ancient tenure of inheritance which they called the tenure of the straw. The Lord's officers would not allow it, because of the breaks that had been made in that manner of holding by the granting of leases, a change dated by one nineteenth-century editor to 1643 and laid at the door of the seventh Earl. Delegates of the Keys made proposals to the tenth Earl on behalf of themselves and all the tenants, and those proposals, slightly modified, became the Act. Estates were declared good and perfect customary estates of inheritance, descendable from ancestor to heir, in return for fines certain on descent and on alienation and the accustomed rents, boons, suits and services. The Lord expressly reserved his royalties, regalia, prerogatives, homages, fealties, escheats, forfeitures, mines and minerals, quarries, franchises and jurisdictions.

Bishop Wilson's part in the negotiation is recorded in his own diary. On 6 September 1703 he wrote that he had been, he hoped, a happy instrument in bringing the Lord of Man and his people to an agreement, his Lordship having that day condescended to settle them upon a certain tenure, or rather to restore them to their ancient tenure, which had been uncertain for more than a hundred years.

The Act did not lapse at Revestment. One of the earliest statutes passed after the Crown purchased the sovereignty in 1765 was an Act for the more effectual confirmation and establishment of the Act of Settlement and the Act of Tynwald explanatory thereof, which declared both to be the basis of the tenure of lands in the Island. It was promulgated on 9 September 1777, and the royal approbation was signified from St James's on 21 August 1777 in a letter from Suffolk returning the Acts transmitted by Governor Smith. The Act is repeatedly described in the Manx Society volumes as the Magna Charta of the Island as between the Lords and their tenants.