In this master thesis, we study possible effects of the scalar leptoquark $S_1$ in top quark decays. The motivation for such an extension of the Standard Model comes from deviations observed in decays of $B$ mesons, in particular in the ratios $R_{D^{(*)}}$, which indicate a possible violation of lepton flavour
universality. Leptoquarks are especially interesting in this context because they couple simultaneously to quarks and leptons and can therefore contribute to semileptonic processes already at tree level. In this work, we focus on the scalar leptoquark model $S_1$ with a minimal number of Yukawa couplings $y^L_{b\tau}$ and $y^R_{c\tau}$. Assuming the leptoquark mass $m_{S_1}=1.5 \text{TeV}$, we integrate it out at the energy scale relevant
for top quark decays and describe its low-energy effects with effective dimension-six operators. The resulting Wilson coefficients are then used to calculate decay widths and branching ratios for the processes $t\to c\tau^+\tau^-$ and $t\to b\tau^+\nu_\tau$. For the rare decay $t\to c\tau^+\tau^-$, we find a branching ratio of order $10^{-8}$, which is
much larger than the Standard Model prediction. In the decay
$t\to b\tau^+\nu_\tau$, the leptoquark contribution appears as a small correction to the dominant Standard Model contribution, where the interference term plays the most important role.
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