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The light curve of the microlensing event KMT-2021-BLG-0240 exhibits ashort-lasting anomaly with complex features near the peak at the 0.1~mag levelfrom a single-lens single-source model. We conducted modeling of the lensinglight curve under various interpretations to reveal the nature of the anomaly.It is found that the anomaly cannot be explained with the usual model based ona binary-lens (2L1S) or a binary-source (1L2S) interpretation. However, a 2L1Smodel with a planet companion can describe part of the anomaly, suggesting thatthe anomaly may be deformed by a tertiary lens component or a close companionto the source. From the additional modeling, we find that all the features ofthe anomaly can be explained with either a triple-lens (3L1S) model or abinary-lens binary-source (2L2S) model obtained under the 3L1S interpretation.However, it is difficult to validate the 2L2S model because the light curvedoes not exhibit signatures induced by the source orbital motion and theellipsoidal variations expected by the close separation between the sourcestars according to the model. We, therefore, conclude that the twointerpretations cannot be distinguished with the available data, and either canbe correct. According to the 3L1S solution, the lens is a planetary system withtwo sub-Jovian-mass planets in which the planets have masses of0.32--0.47~$M_{\rm J}$ and 0.44--0.93~$M_{\rm J}$, and they orbit an M dwarfhost. According to the 2L2S solution, on the other hand, the lens is a singleplanet system with a mass of $\sim 0.21~M_{\rm J}$ orbiting a late K-dwarfhost, and the source is a binary composed of a primary of a subgiant or aturnoff star and a secondary of a late G dwarf. The distance to the planetarysystem varies depending on the solution: $\sim 7.0$~kpc according to the 3L1Ssolution and $\sim 6.6$~kpc according to the 2L2S solution.

KMT-2021-BLG-0240: Microlensing event with a deformed planetary signal