The arrested man was identified as Willy Nauenartwanich, who is included on an Interpol list as a suspect wanted by India for terrorism offences, the sources said.
Nauenartwanich has denied his involvement in gun running.
The next step could be his extradition to India, sources told PTI but did not elaborate.
It could not immediately be ascertained where or when Nauenartwanich was arrested.
The outlawed NSCM-IM operated out of Bangkok and several other Thai cities till about a decade ago but most cadres of the group have reportedly moved out of the country.