The subtle interplay between competing degrees of freedom, crystal electric fields, and spin correlations can lead to exotic quantum states in $4f$ ion-based frustrated triangular lattice antiferromagnets. We present the crystal structure, thermodynamic and muon spin relaxation ($\mu \mathrm{SR}$) studies of the $4f$ ion-based frustrated magnet ${\mathrm{Ba}}_4{\mathrm{YbReWO}}_{12}$, wherein ${\mathrm{Yb}}^{3+}$ ions constitute a triangular lattice. The magnetic susceptibility does not show any signature of spin freezing down to 1.9 K or long-range magnetic ordering down to 0.4 K. The low-temperature Curie-Weiss fit to the inverse magnetic susceptibility data reveals a weak antiferromagnetic exchange interaction, which is corroborated by the fit of magnetic specific heat data following the $J_1-J_2$ model with the nearest-neighbor-exchange interaction of $J_1\approx -0.197\mathrm{K}$ between the $J_{\mathrm{eff}}$ = 1/2 states of the ${\mathrm{Yb}}^{3+}$ moments in the lowest Kramers doublet. The lowest Kramers ground state doublet is well separated from the first excited state with a gap of ${\mathrm{\Delta }}_{\text{CEF}}$ = 278 K, as evidenced by our $\mu \mathrm{SR}$ experiments that support the realization of $J_{\text{eff}}=1/2$ at low temperatures. The specific heat experiments do not detect a phase transition down to 56 mK. The magnetic specific heat shows a broad maximum at 90 mK suggesting a disordered ground state with short-range spin correlations. The associated magnetic entropy release at low temperatures is consistent with that expected for the $J_{\mathrm{eff}}$ = 1/2 state. The zero-field $\mu \mathrm{SR}$ measurements show neither the signature of spin freezing nor a phase transition, at least down to 43 mK. Our results suggest a dynamic, disordered ground state in this $J_{\mathrm{eff}}$ = 1/2 frustrated triangular lattice antiferromagnet. ${\mathrm{Ba}}_{4}R{\mathrm{ReWO}}_{12}$ ($R$=rare earth) offers a viable platform to realize intriguing quantum states borne out of spin-orbit coupling and frustration.

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• Frustrated magnetism
• Magnetic susceptibility
• Magnetism
• Specific heat
• Antiferromagnets
• Muon spin resonance