In the law of the Republic of Serbia, the cause of contractual obligation has the significance of a general assumption of the validity of the contract. It represents an essential condition for the creation and survival of every contract. Accordingly, “every contractual obligation must have a permissible cause. The cause is impermissible if it is contrary to cogent rules, public policy, or good morals” (Article 51(1-3) of Law on Obligations). It is assumed that the contractual obligation has a cause, even though it is not expressed. The legal consequence of the non-existence or impermissibility of the cause is the nullity of the contract. The subject of interest in this paper is the consideration of peculiarities regarding the cause of obligation of the contract on representation in sports, which distinguish it from other types of contracts. For that purpose, special attention is devoted to the analysis a few characteristic cases of concluding contracts on representation in sport in contractual practice. It has been observed that existence of false (unreal) cause in the practice of formation of these contracts are not rare. Additionally, the unreal ground is, as a rule, the result of the will of the contractors themselves. The existence of a putative cause in contracts on representation in sports is difficult to imagine due to the mandatory written form prescribed by law. Therefore, the false cause in these contracts can be manifested in the form of a fictitious or simulated contract. The falseness of the ground is most often expressed in the form of a reduction or increasing of the obligations stated in the written contracts, compared to those that were actually contracted. It would be difficult to say that such payments, outside of legal payment flows, are an unusual phenomenon in modern sports. Considering the above, it is necessary to take a position regarding the solution of the exposed legal problem from the point of view of national legislation. On the ground of the rules contained in the legislation of the Republic of Serbia, the author concludes that the existence of an false cause results in the nullity of all contracts that were used in the simulation operation. Therefore, the sanction would consist in the absolute nullity of the fictitious, simulated and dissimulated contract due to the circumvention of legal regulations, the mutual confiscation of the object of performance due to violation of cogent regulations and public policy, as well as compensation for damage caused due to the violation of legally protected interests of bona fide third parties.