Overall water splitting driven by a low voltage is crucial for practical H2 evolution, but it is challenging. Herein, anion‐modulation of 3D Ni–V‐based transition metal interstitial compound (TMIC) heterojunctions supported on nickel foam (Ni3N‐VN/NF and Ni2P‐VP2/NF) as coupled hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts for efficient overall water splitting is demonstrated. The heterointerface in Ni3N‐VN has a suitable H* absorption energy, being favorable for enhancing HER activity with onset overpotential (ηonset) of zero and Tafel slope of 37 mV dec−1 in 1 m KOH (close to that of Pt/C/NF). For the OER, the synergy of Ni2P‐VP2 with oxide species can give enhanced activity with ηonset of 220 mV and Tafel slope of 49 mV dec−1. The good activity is ascribed to heterointerface for activating the intermediates, good conductivity of TMICs for electron‐transfer, and porous structure facilitation of mass‐transport. Additionally, the minimal mutual influence of Ni3N‐VN/NF and Ni2P‐VP2/NF allows easy coupling for efficient overall water splitting with a low driving voltage (≥1.43 V), a voltage of 1.51 V at 10 mA cm−2, and remarkable durability for 100 h. It can be driven by a solar cell (1.5 V), indicating its potential to store intermittent energy. An anion‐modulation strategy is presented to create 3D Ni–V interstitial compound heterojunctions (Ni3N–VN/nickel foam (NF) and Ni2P‐VP2/NF). The excellent hydrogen evolution reaction (HER) activity of Ni3N–VN/NF and oxygen evolution reaction (OER) activity of Ni2P–VP2/NF and minimal mutual influence make them easy to couple to achieve overall water splitting with a low driving voltage and remarkable stability.