Long-range order in quasi-one-dimensional (q1D) arrays of superconducting nanowires is established via a dimensional crossover from a fluctuating 1D regime to a phase-coherent 3D ground state. If a homogeneous crystalline superconductor exhibits sufficiently high uniaxial anisotropy, a similar 1D$→$3D crossover has been predicted to occur, provided that single-particle hopping transverse to the 1D axis is absent in the normal state. Here we present magnetic penetration depth and electrical transport data in single crystals of q1D Tl$_2$Mo$_6$Se$_6$, which reveal a 1D$→$3D superconducting dimensional crossover. Both experimental techniques uncover multiple energy scales within the superconducting transition, which describe a sequence of fluctuating regimes. As the temperature is reduced below $T_{ons}=$~6.7~K, 1D pairing fluctuations are replaced by 1D phase slips below $T_p∼$~5.9~K. These give way to 3D phase fluctuations below $T_{ab}=$~4.9~K, prior to dimensional crossover at $T_{x2}∼$~4.4~K. The electrical resistivity below $T_{ab}$ is quantitatively consistent with the establishment of phase coherence through gradual binding of Josephson vortex strings to form 3D loops. An anomalously low superfluid density persist down to $∼$3~K before rising steeply --- in agreement with a theoretical model for crossovers in q1D superconductors, and suggesting that a small population of unbound, weakly-pinned vortices survives below the crossover. The observation of a dimensional crossover within the superconducting state has important consequences for the low-temperature normal state in Tl$_2$Mo$_6$Se$_6$ and similar q1D metals, which may exhibit one-dimensional behavior over far greater temperature ranges than band structure calculations suggest.