A novel ternary compound, UMo3B7, has been synthesized by arc melting and annealing at 900 C. Its crystal structure was determined from X-ray single crystal diffraction data (YMo3B7-type structure, space group Pnma; a=1.10310(8) nm, b=0.30995(2) nm, c=1.2792(1) nm, RF2=0.0205). The structure is composed of boron filled trigonal prisms as well as unfilled tetrahedra and tetragonal pyramids formed by metal atoms. With respect to boron atoms aggregation, it exhibits a well-developed two-dimensional boron network revealing infinite bands of edge linked boron hexagons. The relationship with the members of the structural series within the V-B system: VnBn+1= (n-1)VB (CrB-type) + VB2 (AlB2-type) (n=1, 2, 3, 5) is discussed. Specific heat, magnetic susceptibility and electrical resistivity measurements characterizes UMo3B7 as a spin fluctuating system.