Superconducting proximity effects in topological insulators enable novel phenomena such as the Josephson diode effect (JDE), yet disentangling surface and bulk contributions remains challenging. Here we report a tunable JDE mediated by topological surface states in ${\mathrm{BiSbTeSe}}_2$ (BSTS) Josephson junctions. Finite-momentum Cooper pairing of spin-helical surface states gives rise to a pronounced JDE under an in-plane magnetic field, with diode efficiency reaching up to 40%. Temperature-dependent measurements reveal that at higher temperatures, the supercurrent is dominated by surface states, resulting in a rapid increase in diode efficiency upon cooling, while the onset of bulk contributions at lower temperatures leads to efficiency saturation. Gate tuning approaching the Dirac point region enhances surface transport, increasing the diode efficiency from 40% to 50%. These results provide strong evidence for a dominant role of topological surface states in the nonreciprocal superconductivity and establish BSTS junctions as a promising platform for tunable, high-performance superconducting diodes.

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• Electrical properties
• Josephson effect
• Magnetotransport
• Quantum transport
• Surface states
• Topological insulators
• Topological materials