This article seeks to explore the issue of the required form of agreement on the transfer of rights and obligations of a party to a contractual relationship by a third party, which in the Polish legal system functions as an innominate contract and is more common in foreign legislations (Vertragsübernahme, transfer of contract, cession de contract). This issue has been analysed taking into account the various possible configurations of parties to such an agreement as well as the various legal natures of the rights and obligations transferred. So far, this matter has not been thoroughly discussed in the Polish legal literature. According to the author, in line with the unity theory, an agreement on the transfer of rights and obligations of a party to a contractual relationship should be classified as a single legal transaction and not as two separate (independent) transactions of claim assignment (Article 509 of the Civil Code) and debt transfer (division theory). Therefore, it requires a written form otherwise being invalid. This requirement applicable to an entire agreement results from Article 522 sentence 1 of the Civil Code; it also meets less strict formal requirements regarding claim assignment (Article 511 of the Civil Code). In this paper, the author also discusses the question of whether Article 77 § 1 of the Civil Code and pactum de forma regarding amendments to an agreement (Article 76 sentence 1 of the Civil Code) apply to the form of the agreement in question.