The green phase compounds $R_2\mathrm{BaCu}{\mathrm{O}}_5$ ($R=\mathrm{Rare}$ earth) have recently emerged as promising candidates for magnetoelectric coupling arising from $4f\text{−}3d$ exchange interactions. Here, we report the discovery of multiferroicity in a previously unexplored magnetic phase of $\mathrm{Y}{\mathrm{b}}_2\mathrm{BaCu}{\mathrm{O}}_5$. While earlier studies identified two antiferromagnetic (AFM) transitions below 15 K, our comprehensive investigation using magnetic, heat capacity, and neutron diffraction reveals three successive magnetic transitions occurring at $T_{\mathrm{N}1}=15.9$ K, $T_{\mathrm{N}2}=5.2$ K, and $T_{\mathrm{N}3}=3.5$ K. Neutron diffraction study evidences a sequence of magnetic phases: a transition from a paramagnetic state to (i) a first commensurate (CM) AFM phase with ${\mathbf{k}}_{\mathbf{1}}=\left(0,{}^1/{}_2,0\right)$, below $T_{\mathrm{N}1}$, (ii) an incommensurate (ICM) AFM phase with ${\mathbf{k}}_{\mathbf{2}}=(0,\delta ,0)$, below $T_{\mathrm{N}2}$, and (iii) a second commensurate AFM phase with ${\mathbf{k}}_{\mathbf{3}}=\left(0,{}^1/{}_2,{}^1/{}_2\right)$ below $T_{\mathrm{N}3}$. Notably, multiferroic behavior emerges exclusively in the incommensurate AFM region, consistent with the deduced polar magnetic symmetry $2mm$. These results establish $\mathrm{Y}{\mathrm{b}}_2\mathrm{BaCu}{\mathrm{O}}_5$ as a new spin-driven multiferroic and underscore the critical role of $4f\text{−}3d$ coupling in governing the magnetoelectric properties of the $R_2\mathrm{BaCu}{\mathrm{O}}_5$ family.

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• Electric polarization
• Ferroelectricity
• Magnetism
• Magneto-dielectric effect
• Cuprates
• Magnetization measurements
• Neutron diffraction