  We report on the Tomonaga-Luttinger liquid (TLL) behavior in fully degenerate
1D Dirac fermions. A ternary van der Waals material Nb$_9$Si$_4$Te$_{18}$
incorporates in-plane NbTe$_2$ chains, which produce a 1D Dirac band crossing
Fermi energy. Tunneling conductance of electrons confined within NbTe2 chains
is found to be substantially suppressed at Fermi energy, which follows a power
law with a universal temperature scaling, hallmarking a TLL state. The obtained
Luttinger parameter of ~0.15 indicates strong electron-electron interaction.
The TLL behavior is found to be robust against atomic-scale defects, which
might be related to the Dirac electron nature. These findings, as combined with
the tunability of the compound and the merit of a van der Waals material, offer
a robust, tunable, and integrable platform to exploit non-Fermi liquid physics.
