Dimerized quantum magnets provide fertile ground for exploring novel quantum phases arising from the condensation of magnon excitations. Here, we investigate the quantum dimer magnet Cu(en)${}_2{\mathrm{SO}}_4$ using specific heat, magnetocaloric effect, and electron spin resonance (ESR) measurements, combined with infinite projected entangled pair state (iPEPS) simulations. Our results establish a dimer-singlet ground state in Cu(en)${}_2{\mathrm{SO}}_4$ with intradimer coupling $J\approx 10.9\mathrm{K}$. A magnetic field applied along the $c$ axis drives two quantum phase transitions at $B_{\mathrm{c}1}\approx 6.43\mathrm{T}$ and $B_{\mathrm{c}2}\approx 11.73\mathrm{T}$, enclosing a dome-shaped antiferromagnetic phase below $T_{\mathrm{c}}\simeq 0.65\mathrm{K}$. This ordered state is naturally interpreted as a result of triplon Bose-Einstein condensation (BEC), with the excitation gap closing and reopening across the critical fields. ESR spectra reveal not only sharp magnon resonances, but also broad continuumlike features, pointing to unconventional spin dynamics that may arise from triplet excitations. iPEPS simulations reproduce the experimental critical fields and confirm a stripe-type spin configuration with ordering wave vector at the $\mathrm{\Gamma }$ point, establishing Cu(en)${}_2{\mathrm{SO}}_4$ as a clean platform for studying magnon BEC and field-tuned quantum criticality.

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• Antiferromagnetism
• Critical phenomena
• Excited-state quantum phase transitions
• Magnetic anisotropy
• Magnetic coupling
• Magnetic interactions
• Magnetic phase transitions
• Magnetic susceptibility
• Phase diagrams
• Phase-change materials
• Quantum criticality
• Second order phase transitions
• Spin dynamics