O One of the most commonly used forms of security in credit granting is mortgage right (hak tanggungan). the issues examined in this study concern whether the substitution of the collateral object under a mortgage right is in accordance with the applicable legal provisions and the legal consequences of such substitution as reflected in decision number 901/pdt.g/pn.jkt.sel. this research employs normative legal research with a descriptive-analytical nature. the data used consist of secondary data obtained through library research, supported by primary legal materials. the analysis is conducted qualitatively, and conclusions are drawn using the deductive method. the substitution of the collateral object under a mortgage right refers to the accessory principle in the credit agreement; however, the substitution of right to build title (hgb) number 920/grogol utara sub-district with a bellagio apartment penthouse unit could not be implemented due to the failure to fulfill the requirement of mutual consent between the parties as stipulated in article 1320 of the indonesian civil code. as a legal consequence, bank bni, as the holder of the mortgage right, remains entitled to execute the collateral, and based on the principle of droit de suite, third-party ownership claims do not extinguish the mortgage right, as such right continues to follow the object regardless of whose possession it is in.