We synthesized a violet-blue phosphor ${\mathrm{Ba}}_2\mathrm{Al}{\mathrm{B}}_4{\mathrm{O}}_9\mathrm{Cl}\text{:}{\mathrm{Eu}}^{2+}$ with a solid-state reaction. The excitation and emission spectra of this phosphor showed that all were broadband due to $4f^7-4f^6{d}^1$ transitions of ${\mathrm{Eu}}^{2+}$ . The phosphors with different ${\mathrm{Eu}}^{2+}$ concentrations presented violet-blue luminescence for ultraviolet [(UV) $250–390\mathrm{nm}$ ] excitation. The optimum concentration of ${\mathrm{Eu}}^{2+}$ in ${\mathrm{Ba}}_2\mathrm{Al}{\mathrm{B}}_4{\mathrm{O}}_9\mathrm{Cl}\text{:}{\mathrm{Eu}}^{2+}$ is determined to be $6\mathrm{mol}\mathrm{.}\%$ . The luminous efficiency was found to be $8.1\mathrm{lm}/\mathrm{W}$ for the violet-blue fluorescent lamp and $3.2\mathrm{lm}/\mathrm{W}$ for the violet-blue phosphor-converted light-emitting diode, respectively. ${\mathrm{Ba}}_2\mathrm{Al}{\mathrm{B}}_4{\mathrm{O}}_9\mathrm{Cl}\text{:}{\mathrm{Eu}}^{2+}$ would be a promising phosphor for converting the UV radiation to violet-blue emission for a novel high light-conversion efficiency phototherapy illuminator.