  The light curve of the microlensing event KMT-2021-BLG-1898 exhibits a
short-term central anomaly with double-bump features that cannot be explained
by the usual binary-lens or binary-source interpretations. With the aim of
interpreting the anomaly, we analyze the lensing light curve under various
sophisticated models. We find that the anomaly is explained by a model, in
which both the lens and source are binaries (2L2S model). For this
interpretation, the lens is a planetary system with a planet/host mass ratio of
$q\sim 1.5\times 10^{-3}$, and the source is a binary composed of a turn off or
a subgiant star and a mid K dwarf. The double-bump feature of the anomaly can
also be depicted by a triple-lens model (3L1S model), in which the lens is a
planetary system containing two planets. Among the two interpretations, the
2L2S model is favored over the 3L1S model not only because it yields a better
fit to the data, by $\Delta\chi^2=[14.3$--18.5], but also the Einstein radii
derived independently from the two stars of the binary source result in
consistent values. According to the 2L2S interpretation, KMT-2021-BLG-1898 is
the third planetary lensing event occurring on a binary stellar system,
following MOA-2010-BLG-117 and KMT-2018-BLG-1743. Under the 2L2S
interpretation, we identify two solutions resulting from the close-wide
degeneracy in determining the planet-host separation. From a Bayesian analysis,
we estimate that the planet has a mass of $\sim 0.7$--0.8~$M_{\rm J}$, and it
orbits an early M dwarf host with a mass of $\sim 0.5~M_\odot$. The projected
planet-host separation is $\sim 1.9$~AU and $\sim 3.0$~AU according to the
close and wide solutions, respectively.
