A new valence precise Zintl phase, Pr4MnSb9, has been successfully synthesized by solid-state reaction at high temperature. The single-crystal X-ray diffraction data reveal its monoclinic symmetry in the space group C2/m (No. 12) with a = 24.12(2) angstrom, b = 4.203(3) angstrom, c = 15.67(2) angstrom, beta = 98.05(1)degrees, and Z = 4. The structure is characterized by the covalent three-dimensional network constructed by two types of five-atom-wide Sb-5(7-) ribbons that are joined by 6-fold coordinated Mn3+ cations, through which the narrower three-atom-wide Sb-3(5-) ribbons are attached as a tag, and interstitial Pr3+ cations and single Sb3- anions locate within the tunnels. Its magnetic susceptibility and isothermal hysteresis suggest, ferrimagnetic behavior. The electrical conductivity and Seebeck coefficient of the cold-pressed pellet suggest a semimetal feature that agrees with the spin-polarized calculation results using the tight-binding linear muffin-tin orbital (TB-LMTO) method.