Trillium lattices, where magnetic ions form a chiral network of corner-sharing triangles, offer a three-dimensional magnetic framework that can host fragile classical spin-liquid states. Herein, we report on the magnetization, specific heat, electron spin resonance (ESR), and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ of ${\mathrm{K}}_2{\mathrm{Fe}}_2\left({\mathrm{MoO}}_4\right){\left({\mathrm{PO}}_4\right)}_2$ single crystals. Magnetization measurements reveal strong antiferromagnetic interactions coexisting with weak magnetic order at $T_{\mathrm{N}}=5.2\mathrm{K}$, as evidenced by a $\lambda$-like anomaly observed in the magnetic susceptibility, a critical enhancement of the muon spin relaxation rate and the wipeout of the ESR signal as the temperature approaches $T_{\mathrm{N}}$. Above $T_{\mathrm{N}}$, two distinct developments of short-range spin correlations are identified at $T_{\mathrm{H}}=34\mathrm{K}$ and $T_{\mathrm{L}}=10\mathrm{K}$, supported by magnetic specific heat anomalies and the temperature dependence of the ESR linewidth and $g$ factor. Upon cooling below $T_{\mathrm{N}}$, an anomaly appears at $T^{*}=3.2\mathrm{K}$ in thermodynamic observables and the muon spin relaxation rate, indicative of spin reorientation driven by residual interactions. Despite the presence of magnetic order, $\mu \mathrm{SR}$ experiments reveal dynamically fluctuating spins persisting even in the ordered state. Moreover, the suppression of $T_{\mathrm{N}}$ under applied magnetic fields $({\mu }_{0}H\ge 2\mathrm{T})$ suggests that ${\mathrm{K}}_2{\mathrm{Fe}}_2\left({\mathrm{MoO}}_4\right){\left({\mathrm{PO}}_4\right)}_2$ constitutes a promising candidate for exploring field-induced spin-liquid behavior in three-dimensionally coupled trillium lattices.

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• Magnetic order
• Magnetism
• Antiferromagnets
• Magnetic systems
• Strongly correlated systems
• Electron spin resonance
• Muon spin resonance