  Magnetic excitations in the spin chain candidate Sr$_2$V$_3$O$_9$ have been
investigated by inelastic neutron scattering on a single crystal sample. A
spinon continuum with a bandwidth of $\sim22$ meV is observed along the chain
formed by alternating magnetic V$^{4+}$ and nonmagnetic V$^{5+}$ ions.
Incipient magnetic Bragg peaks due to weak ferromagnetic interchain couplings
emerge when approaching the magnetic transition at $T_N\sim 5.3$ K while the
excitations remain gapless within the instrumental resolution. Comparisons to
the Bethe ansatz, density matrix renormalization group (DMRG) calculations, and
effective field theories confirm Sr$_2$V$_3$O$_9$ as a host of weakly coupled
$S = 1/2$ chains dominated by antiferromagnetic intrachain interactions of
$\sim7.1$(1) meV.
