Kevin Sorbo -- Sued By Russian Promoter ... Hercules Is a Cheap-nik

"Hercules" star Kevin Sorbo used a Russian publicist to get face time with lots of rich Russian investors ... and then stiffed her, so now she's suing.

Sophie Bardo says Sorbo hired her to organize a tour of Russia in December 2013, and says she arranged meetings, interviews and public appearances for him ... according to court docs obtained by TMZ.

Bardo -- publicist by trade, Russian by descent -- says the purpose of the trip was to promote Sorbo's image in Russia and find financial backers for his TV shows and movies.

Bardo claims Sorbo has refused to pay her -- even though she's been doing this kind of work for him since 2009. That's a lotta rubles.