In the present paper, a Ti-39Nb (wt.%) was employed to illustrate the feasibility of designing β-type titanium alloys with low Young's modulus through concurrently reducing shear moduli C44 and C'. The solution treated and quenched alloy exhibits a dual (β+α") phase due to the insufficient Nb content, implying the intrinsically low β-phase stability. After severe cold rolling and annealing treatment at 523 K for 30 min, the martensitic transformation was suppressed by high density of defects. As a result, a nearly single β-phase alloy with a low Young's modulus of 39 GPa and a high ultimate tensile strength of 745 MPa was successfully fabricated. The single-crystal elastic constants of the cold rolled plus annealed Ti-39Nb alloy were extracted from its polycrystalline specimen by using an in-situ synchrotron X-ray technique. It is indicated that the alloy has an anomalously low C44 of 21.4 GPa as well as a low C′ of 17 GPa, both of which contribute to the low Young's modulus. This suggests that it is a feasible approach to concurrently reduce C44 and C′ through composition design and microstructure optimization. Our findings provide a new strategy of designing β-type titanium alloys with ultralow Young's modulus.